Though I loved being an English teacher, my dream from childhood was to be a writer. Aged 34, I was head of English in a secondary school, newly married, and about to start a family. Just three weeks after the wedding I found something strange in my right breast. It was more of a mass than a lump.

On the 11 November 2004 at 1.35pm, I was told I had cancer. The words I remember were “no cure”, “mastectomy”, and “breast cancer and pregnancy don’t mix” – all said in the same sentence.

No one in my family had had breast cancer. I was young, slim, hadn’t eaten meat since I was 12, didn’t smoke, didn’t over-drink, and wasn’t on the pill. The bad news got worse: I had the most aggressive form of breast cancer - HER2. The tumour was 7cm. It had spread to my lymph glands and a blood vessel in my chest. I’d need radical surgery, chemotherapy and radiotherapy.

I had a mastectomy. Nothing can prepare you for how this looks. When I saw mine the day after the op, I passed out cold. Even now I have a very bony chest wall on my right side. My treatment was life-saving but also life changing, a constant reminder of what I’ve been through. Then chemo. Ugh, chemo.

It makes you gain weight. It makes you tired beyond reason. It makes you forget things. Gives you ulcers. Makes you catch every bug going for years afterwards. And the hair loss: I wasn’t worried about it until it happened. I had short hair anyway, and in my student days had once or twice shaved my head. But again, nothing can prepare you for it. Hair loss is so public. So obvious.

And not just head hair, but eyebrows and eyelashes too. No hat, wig or headscarf can hide the fact that you are A CANCER PATIENT.

Much more positive was my experience with newer medicine. When my breast nurse asked the consultant about Herceptin, the reply was: “Not yet.” The chances of HER2 recurring was high – 50/50. Herceptin was already being used to treat secondary breast cancers. It was a new type of drug that triggered the body’s own immunity to fight the tumour. It wasn’t yet available on the NHS in the UK, but some private patients were being offered it.

Six months after my diagnosis, my cousin was also told she had HER2 breast cancer. Her husband’s private medical insurance paid for her to have Herceptin. The treatment still wasn’t available to me, or people like me. After much soul-searching, I decided to try to fund it myself – at the cost of more than £30,000.

Then Barbara Clark, an HER2 cancer patient from Somerset, challenged her primary care trust’s decision not to fund Herceptin when it clearly had clinical benefits. After threatening them with court, they agreed to pay. Her own fundraising had amassed thousands of pounds, which she donated to other HER2 patients, myself included. By November of 2005, Patricia Hewitt, the then health secretary, finally ruled that Herceptin could be given to all HER2 patients, regardless of their trust.

It was both incredible and awful to be at the cutting edge of this controversy. Had I been diagnosed even six months earlier, I might well have missed out on Herceptin. In a period of terrible luck, I experienced some very good fortune. I’m convinced Herceptin saved my life.

Fast-forward four years, and my cancer treatment was over. Yet the life I’d had before cancer wasn’t there any more; I wasn’t the same person either. I needed something to fill what cancer had taken. Luck played a part in this too.

In the summer of 2009 I took a group of students on an Arvon residential course. I started writing. And writing. When I came home six days later, I couldn’t stop crying. Or writing. So I signed up for a master’s in writing for children.

What started out as a childhood dream quickly grew into a passion. By doing the MA, I felt I’d validated my writing, given myself “permission” to take it seriously. I tapped into something long hidden inside of me and brought it out again, fresh and new.

Cancer has taken away my chance to be a mother, changed my career path, and made my future questionable. Yet it’s made me rethink life dramatically. And now I’m doing the one thing I always wanted to do – write books. My fifth novel is about to be published. The nightmare has happened, but then so has the dream.

• Emma’s new novel Strange Star, inspired by Mary Shelley’s Frankenstein, will be released on 7 July from Faber & Faber.