A few years ago, when I lived in south London, I would walk down Brixton Hill every day to get to the tube station, past several women waiting on the street for a car to pull up alongside them. They would be there in the morning and when I came home at night. One of them was Rhea Coombs. I think I can remember her - she had short, bleached blonde hair and her clothes hung off her - but maybe it's because no matter what they were wearing, they all looked the same, all hollow faces and shivering limbs.

Coombs has now been clean for nearly three years and has written a book about her experience of prostitution and drug addiction. She says one of the reasons she chose to write it was to give people an insight into the lives of street sex workers and to make people understand how vulnerable and damaged most are - a point graphically underscored by the murders of five young women in Ipswich just over a month ago. "These women are so dehumanised," she says. "It really upset me when I watched the news and they kept being referred to as prostitutes rather than women. It was so degrading. These women shouldn't be criminalised. That's not helping. There should be areas where prostitution is decently policed and where it's allowed and can be safer for women. If it's kept secret and dark, where it gets pushed into corners where nobody can see what's going on, that's when the bad stuff starts happening."

It could be an elaborate shell, but despite everything she has been through - violent relationships, losing her children, getting involved with gangsters, addiction and the gruelling way she paid for it - there doesn't seem to be any sign of "the victim" about Coombs. Self-destructive she might have been, but she was also tough. At one point, she was running a crack house, virtually unheard of for a woman; she writes that the pity she got from some of her punters irritated her, when all she wanted was their money.

Coombs lives in a neat terraced house in south London with her two children. Her nine-year-old daughter is upstairs in her bedroom, singing to a karaoke machine, and her son, a tall, handsome boy of 16, bounds out of the door with his friends. Was she worried about how they would feel about the book? "My son seems quite happy about it but has made it clear he doesn't want to read it. I was a bit more worried about [how] my family [would react] but some of my old behaviour was about worrying what people thought of me, whether they would accept me as me. That has changed."

Coombs had her first experience of drugs and prostitution as a young teenager. Her childhood had been spent following her bohemian mother around Europe, from commune to commune. When they eventually settled in Bristol, Coombs - who says she always felt like an outsider - reinvented herself as the hard girl at her new school and discovered cannabis. "It was like, wow, I've come home," she says. "It always felt like [taking drugs] was the thing for me, that it was wonderful. I spent most of my life with a huge vacant hole inside me, that's the only way I can think of to describe it." The drugs helped, but not for long. "Sure enough, the same feeling comes back like an eternal irritation that can't be removed."

Desperate to grow up and create a stable life for herself, she became involved with an older man who persuaded her to contribute to their income by having sex with men for money. At the age of 14, she found herself in the back seat of a car with a fat, bald man while the woman who had arranged it sat in the front. Then, it was a one-off. How did that early experience affect her? "I knew what the end situation [earning money and keeping her boyfriend] would be so that's how I saw it," she says, matter-of-factly. That relationship didn't last, and she fell into other relationships with other men who became abusive. At 16, she had her son. One of the men she got involved with beat her so badly that she could hardly walk. Coombs escaped to London, but became involved with another man who would torment her and her son with a knife, and kept them prisoner in their flat, refusing to let them eat. One time, he tied her small son to a chair and repeatedly punched him in the face while Coombs could only look on, screaming. They escaped again, climbing out of an unlocked window.

She says writing the book was a kind of therapy but there are still incidents, like the one with her son, that she finds it difficult to talk about. "It did help," she says. "But there's still stuff in there that I haven't dealt with, which need to be." One is an incident of sexual abuse in her childhood.

Coombs says her lack of self-esteem made her an easy target for these men. "I was so blinded by this experience of not feeling worthwhile and questioning why I was even alive. It makes you feel worthwhile when someone pays you a bit of attention." By the time she had her daughter, with another man, Coombs was working as a hairdresser, a job which supported her and the children once her relationship broke down. Her drug-taking escalated. What had once been a largely weekend habit - speed and ecstasy in nightclubs - started to encroach on the week. And she discovered crack.

For several years she managed to maintain her life like this - she would get her children to school on time and smoke crack before it was time to pick them up - but eventually, her addiction became too much. Her daughter's father took both the children away and the pressure of trying to keep it together at the salon became too much. "I had presented this facade to everyone for a long time, this fun- loving, crazy, clubbing, got-it-together mother who could swallow loads of drugs but still have a stable life. I knew the facade was breaking and it became too stressful to hold it together."

From there to prostitution suddenly didn't seem like such a huge leap. By her late 20s, she would be out on the streets day and night picking up punters for £20 a time, which paid for rocks of crack and, later, heroin. "I felt freer, more comfortable. I enjoyed being out in the dark because it was almost as if the person that I was during the day completely disappeared. I felt like I belonged." Did she feel that people, like me on my way to work, judged her? "It was almost as if people couldn't bring themselves to actually look at you so you feel completely obscured into the background."

In her book she describes the men who paid to have sex with her. "Some of my punters were rich, others were poor, some smelled of clean washing and cologne, but most of stale beer, cigarettes and sweat." She writes that prostitution sometimes made her feel empowered - that she was getting paid for something their wives or girlfriends weren't, that men to her became little more than their urges, and by pretending to be coy and vulnerable she was fooling them. "They think they've got all the power," Coombs says. "So it becomes that little play, you having to be all coy. They think that they are forcing you to behave in a certain way to please them, that you're the lucky one to be earning their money. But you're the one putting on the show - you don't like the look of them, you don't feel comfortable with them. You become an actress and they fall for it." Mainly, she just hated it.

"You become numb to the experience and all you see is the goal at the end of it. I never enjoyed it but the more I did it, the more it became easier to black it out, to not even be there. You could make it more bearable by collecting regular punters, even if you couldn't stand them, because ultimately they just wanted a piece of you. If it's late and there are police about, you can't afford to be choosey so you get on with it."

Coombs wasn't reckless about her safety, but ultimately she didn't really have a choice. "As you get into the car, it's terrifying. Once I was attacked by a man in broad daylight in his car and people were just walking past; they didn't care. Every time you step into someone's car, you think, is this the safe one? Am I going to be all right?' But the money you need for the drugs takes over."

She would work day and night, whenever she needed money to buy drugs. "But I was very conscious around school home time. One of the areas I used to work in was near a school. Even if the parents completely ignore you, the children will stare. And it was too much of a reminder of my own kids." Did she think about them all the time or try to block it out? "I did think about my children but I believed that they were better off not having me in their lives," she says. "I wouldn't go beyond that thought. There were a couple of occasions when I did get back in contact but I couldn't cope with it and that was fuel for the fire. I just felt more hatred for myself."

Coombs tried and failed to come off drugs several times and occasionally got in touch with her family, who urged her to get help. "I had so much pain and hatred inside for myself that was stronger than any family connection," she says. "I hated the fact that it was so obvious to the family. There was no hiding or disguising what was going on with me."

She had been living in a hostel - a place where users took their drugs together and even the cleaners paid the women for sex - so it wasn't surprising that she had failed to cure her addiction. It wasn't until she was given her own flat in east London, with the help of the resettlement officer at the hostel, that she finally managed it. "All my safety nets had gone - the areas I worked in, the people I used with, the dealers I knew," she says. "I was having more contact with my kids but I had so much shame. I would have to time my visits with them and get back out and get some money otherwise I'd start clucking [going through withdrawal]."

A worker from Spires, a charity for the homeless and people with drug addictions, had met Coombs years earlier and stayed in touch with her. With her help, and the discovery of a 12-step meeting, Coombs managed to get clean. "The only way I can describe it is like being sucked into a whirlpool and someone chucks you a lifeline and your only option is to grab it," she says. "I knew I had no other option [other than to come off drugs]. I needed to feel safe. I usually ended up doing what I had to do when I had no other choice. I knew I had run out of excuses. In the first week of being in my new flat, I was going through withdrawal and I had promised my son he could come and stay. He was ringing me up and hassling me and I had to keep pressing cancel on my phone because I couldn't speak to him. I felt so ashamed, the state that I was in, and I didn't want him to see me." It took months of going to meetings, sometimes several in a day, but she managed it. That was three years ago.

Last year, Coombs joined Spires as a women's support worker. Many of the women she now sees are friends from her former life, and her reputation has grown so much that other drug-addicted sex workers ask to see Coombs. "I love my job and I feel very privileged that I've been given the space to work with the women, however I need to, and that these women would trust me with their lives," she says. "I've seen changes [in them] but it takes a long time. One of the reasons I wanted to do the book was to see if I could change anything. Working in the field now, I can see how many brick walls are up against us. It has become impossible to get a woman housed. To be deemed vulnerable and in need of support, it's as if you need to have two limbs missing and be completely incapacitated."

Coombs says her work has helped with her own recovery too. Is she ever tempted by drugs now? "No. Working in the chaos and having it in your face is really draining and a very good reminder that I would never want to go back into that, and also a reminder of how hard it is to get out of [drug addiction]. It's even worse now than when I was out on the street. It's more violent, there are more guns. Recently, one of the girls I see told me she had been in a crack house and a man charged in with dogs, set the dogs off and said, 'I feel like dying tonight.' He switched the lights off and all anyone could see was the silhouette of this man with a gun. Everyone had to beg for their lives to get out."

I wonder how her past, especially the prostitution, affects her now. She says it is difficult, especially when it comes to relationships with men. "I still conflate men and punters," she writes in her book. "I hope that the further away I get from the days of selling my body, the less I'll see men in that way."

"I'm lucky, I have a car," she says now. "One of my things was that I hated travelling on public transport. If a man sat next to me with his legs wide open and his leg touching mine, it would make me outraged. Men invading my personal space, that makes me feel very uncomfortable. I like being on my own and concentrating on what life is all about at the moment." Can she imagine having a relationship again? "I don't try to think too much about the future, that's how I manage life. My self-worth and self-esteem have grown to the point where I don't need anyone else to keep me going. I believe that I had to go through everything for a reason. It wasn't pleasant but I prefer the person I am now since I've been through it. I wouldn't take it all back. I wish I could remove the pain and the hurt I've caused to other people, like my children and family, but for myself, I know I'm better and stronger and able to deal with anything in front of me".

· My Name is Angel, by Rhea Coombs with Diane Taylor, is published by Virgin Books, priced £14.99. To order a copy for £13.99 with free UK p&p go to theguardian.com/bookshop or call 0870 836 0875.