The first surprise is, she is petite. Somehow you expect someone with an appetite like hers to be bigger. Heavier. More, I don't know, solid. And she is poised, and absolutely and perfectly demure in elegant grey skirt, blue silk blouse, classy grey cardie. It is not easy, frankly, to see in her a woman who has had sex with so many different men that she stopped, years ago, trying to put a number on them; so many that they have blurred, she says, into anonymity.

So many hands, so many mouths. Tall men, short men, fat and thin men. Clean, dirty, old, young. In clubs, car parks, saunas, house parties, art galleries. Vans, train stations, stadiums, fields, store rooms, cemeteries. At the biggest swingers' parties she attended, up to 150 men would be present, and she might have sex, in one form or another, with a quarter of them.

"I revelled in it," says Catherine Millet. "It's what I was truly good at – what I was the best at. I loved particularly the anonymity, the abandonment of orgies. The sensation that one was glorying in this unbelievable freedom, this transcendence. I look back on it with nothing but pleasure. It was very important to me, to my identity, my ego, but it wasn't an addiction. I was never a nymphomaniac. I did not pounce on everything that moved. I never provoked. I made myself available. I profited."

She has been profiting since, too. The Sexual Life of Catherine M, Millet's frank, thorough, unemotional and deliberately unerotic account of those adventures, has now been translated into 40 languages and sold more than 5m copies worldwide, making its author – now 60, and still the highly respected editor of a French highbrow contemporary arts magazine that she co-founded more than 30 years ago – if not exactly rich, probably more comfortable than she ever imagined she could be.

But now she is back, and with a very different kind of book. Jealousy relates a three-year period in Millet's life, at a time when she had pretty much given up the orgies but was still indulging in the odd affair, when she realised that her husband, Jacques Henric – who knew all about her infidelities – was also being unfaithful. The discovery knocked her sideways; she describes being unable to sleep or breathe, suffering wild fits of rage, feeling her heartbeat falter. At times she was banging her head against the wall. Not, as she now explains in a light, book-filled living room in Paris's 12th arrondissement, because she was jealous as such, but because she knew she had no right to be.

Some (not me, obviously) might feel moved to remark: that'll teach you. Millet, you see, has always argued that if she led such an extraordinarily indiscriminate sex life it was at least partly because her partnerships – she has only ever actually lived with two men – were so solid. "I had no need," she has written, "to go and build love stories out of sexual relationships." And: "I had love at home. I sought only pleasure outside." So this sudden and vicious attack of "the timeless and universal malady", she explains, was "a real crisis. Physical. I felt like there was no way out; I was living a contradiction. I knew I could never make him understand the pain he was causing me; I could only agree when he said: But how can you possibly reproach me, with the life you've led? Morally very difficult to deal with."

She finally emerged from the depths after two years of therapy, and once she realised that she was deliberately maintaining her jealousy "to derive pleasure from the pain". Because throughout this whole crippling crise, Millet describes herself as being perversely unable to entertain any sexual fantasy whatever – and sexual fantasies are a big part of her life – that did not entail a vision of Jacques coupling vigorously (and, Millet being Millet, graphically) with a mistress. I had these fantasies," she says, "of him and his girlfriends in places we had been to, in positions we used. These are the miracles of the subconscious. Once you become conscious of the mechanisms, they cease to exert such a hold."

It was not an easy process, though, and nor was writing about it. "The Sexual Life of Catherine M took a long time to write," she says, sitting at her living room table, while Jacques works upstairs in his study, "but that was mainly just my own technical difficulty in writing. For Jealousy, I had to make a real effort, not so much to describe the crisis itself, but to relate the way I had behaved. Going through his papers, opening up his drawers, reading his letters – it doesn't exactly cover one in glory, does it? That took me ages. Forever. These are very deep impulses, and they're much more difficult to write about than mere sex."

But why this crisis at that moment? Her career was established; she was successful, curating prestigious exhibitions abroad, admired in her field. She was married to a man (they were together 10 years before marrying, and have now been so for 28) for whom, as she rather quaintly puts it, she had "discovered a real feeling of love". There are several reasons, Millet believes. "It was in the period when I was taking less and less pleasure in orgies. My sex life was always very important for me, for the construction of my personality, the definition of myself. And the discovery that Jacques was having relationships with other women perhaps exacerbated a feeling that I was returning to the state of self-doubt I'd known when I was younger. It's as if I no longer possessed the sexual excellence that was mine when I was young; Jacques had it now. This was his moment, not mine. I imagined him enjoying a pleasure, a privilege, that I had once enjoyed. I suffered more from that than from any fear that he might leave me."

I ask whether Millet feels there is anything in her background that might explain her promiscuity. She was born and grew up in Bois-Colombes, a nondescript suburb west of Paris. Her father was a driving instructor, and her mother experienced periodic bouts of mental illness that eventually terminated in suicide (her brother also died when she was in her early 20s, prompting her to go into analysis for a while). Both her parents had affairs. The apartment was small, and there was precious little money around. She would deny that she ever sought a pleasure denied her in childhood (it was not until she was 35, she claims in The Sexual Life, that it occurred to her that her own pleasure could be the sole objective of a sexual encounter), but it is certainly tempting to see a degree of childhood misery as a psychological driver for her behaviour.

"I don't think so," she says, firmly. "What is true is I could never think that there was some kind of coincidence between love, marriage and sexuality. My parents never bothered to hide their affairs. My mother and grandmother would say of my father when he was away for the weekend, 'He must be off with one of his girlfriends.' And on those weekends my mother's boyfriend would come to lunch. I entered into adult life with no sexual morals whatsoever; I had no notion of rules or taboos. And then – when I was still quite young, in the first few months of my sexual life, soon after I lost my virginity – I found myself in a group, and everyone just started making love. It was supremely pleasant. In the open air. Almost magical, really. And I fell in with people who had multiple sex lives. Simple."

Her mores were also a product of their time; much of what she calls her "sexual family" got up to, she readily concedes, would simply not be possible today. She has, though, recently started thinking about a third book, which would be devoted almost exclusively to a forensic examination of her childhood.

Was her anguished crise de jalousie retribution for the life she had led? "Absolutely not. I could have enjoyed an awful lot less sexual freedom, and still been jealous. I knew people would think that: she thought she was free, she lived the life of a libertine, and now God has punished her . . . It's why I made only very passing reference to the episode in my first book. I wanted that to be a straightforward enumeration of the facts; there was no place for psychology. No place for emotion. This book is all about psychology; all about emotion. It's about the psychology of suffering." (And it's true; Jealousy is as unflinchingly honest as The Sexual Life, but there's barely a rampant organ in sight. Like the emotion it exhaustively dissects, the book takes place – mostly, at least – in the mind. It has had rave reviews in France and I liked it, but then I have a fairly high threshold when it comes to Gallic soul-searching.)

So no regrets, then, Ms Millet? "None at all. I continue to believe that love and sexual desire are feelings you can experience divergently, and that you can be attracted to and love many people at the same time. Of course, there are relationships that are more important, deeper, than others. But there are an infinity of ways in which a person can experience love. We're fighting against the heritage of romanticism, mon ami. I hate giving advice, but we need to rid ourselves of the notion of l'amour unique. It's not like that in real life. Romantic love affairs generally end in tears, you know."

And, um, your husband in all this? On cue, Jacques appears briefly in the stairway. "How's she doing?" he asks. "I hope she's giving you the right answers. You can mark her out of 10, if you like. People do." He disappears again, and she laughs. "You're trying to get me to say he's my grand amour," she scolds. "Well, I did meet the man of my life. I may not have been swept off my feet when I met Jacques, but I did have the impression that . . ." She pauses. "That this was my place." It's only a passing moment of weakness, though, and she collects herself. "The point is," she insists, "that even having a relationship like that doesn't stop you having others. Even from loving others." Indeed.

• Jealousy: The Other Life of Catherine M is published by Serpent's Tail at £10.99. Catherine Millet will be in conversation at the French Institute in London on Thursday 4 November 2009. For tickets go to institut-francais.org.uk or call 020-7073 1350