There is a revered hush in the room as Martha Beck speaks. She is giving a talk in a bookshop in central London and about 40 people have turned up. They listen, rapt, to this slight woman in her mid-40s, and nod when she says things such as "You will never realise your best destiny through the avoidance of fear."

As someone who despises self-help books, Steering by Starlight, Beck's new book, surprised me - Beck, a renowned life coach in the US, is funny and an excellent writer, and she has a healthy degree of self-awareness. "I still can't believe I write self-help books," she says, laughing, when I see her at her hotel the next day. "If my professors at Harvard could have known I would write self-help, I'm not so sure they would have given me my degree."

Beck has had an extraordinary life. She has survived childhood sexual abuse, and removed herself from the Mormon church. She was married for 10 years, before she came out as gay - and only after her husband announced he was homosexual. Now, she is a sought-after life coach, a best-selling writer and appears regularly on Oprah. "Sometimes it's really laughable," she says. "I'll be in a seminar and people will be talking about their experiences and I'll say, 'Oh, that happened to me and that happened to me too.' They think I'm making it up because it's too ridiculous." Where to start?

Beck first came to notice in America when she wrote Expecting Adam, a book about dealing with the birth of her second child, who was diagnosed with Down's syndrome when Beck was seven months pregnant (he is now 20). "The doctors told me it was like having a malignant tumour and not letting them remove it," she says, and she was urged to have an abortion. "My mind went there but my heart couldn't. I had friends who had the same diagnosis who aborted the pregnancy, and I totally support them, but in my case, I knew that wasn't the way I wanted to go. I had to answer the question: what kind of human being is not worth bringing into the world? If it's a flawed human, then we're all mistakes."

It was Adam's diagnosis - which seemed like a huge personal tragedy to Beck at the time - that really changed her life. Beck's life-coaching philosophy seems based on finding and fulfilling your "destiny", which doesn't sound quite so wacky when she explains it. "Most of the time we're off-centre, and we don't feel quite right so we're always seeking something," she says. "We think certain things will give it to us, most of them culturally defined - a lot of money, a bigger house, whatever. Those usually don't fulfil the need, so we keep looking. You play your life like a game of 'getting warmer, getting colder', consistently making choices that make you happier in a deep way. I made that choice with the abortion - that felt 'warmer' to me. Prior to that I had made a lot of choices that felt cold, that I thought would make me happy. With a child who is different, you are constantly confronting it and that's why I became a life coach - it was that gift of having to live with something different and having to make sense of it."

Beck was born in Provo, Utah - the heart of Mormon country - the seventh of eight children. Their father, Hugh Nibley, was a celebrated scholar of the religion. After Adam's birth, Beck and her husband, who was also a Mormon, decided to return to Utah. "I thought that there, people would accept my choice [to have Adam]," she says. She had been teaching at Harvard, where worth was measured by how clever you are, and suddenly she had a child with a severe learning disability; she felt alienated from the people who thought she should have had an abortion. So she went home and threw herself back into her religion. It wasn't long, though, before she started feeling that she had made a mistake. "Mormonism felt so wrong. I guess it had been about three years when I thought this can't go on, then another couple of years of slowly disengaging."

It was after teaching a psychology class at the college where she worked that she decided to leave. "I was behind the glass of a one-way mirror watching the interactions between the students. The whole class got into a discussion of rape and I got more and more agitated. Suddenly I felt I had to run from the room, and I got the door open and then I was hit by this overwhelming pain and I blacked out." She was rushed to hospital and went into surgery. "They found I had a lot of scar tissue [she means vaginal scarring], and I was bleeding internally from old scars. They had to repair a lot of tissue damage."

During the operation, and under general anaesthetic, Beck says she felt some kind of "white-light experience" that gave her an overwhelming feeling of safety and happiness. "I came out thinking, 'that's how I should feel in my life and everything that doesn't feel like that has got to go'," she says. "Within a month, I had rejected my religion, my profession, my job, my family, my home, everything. I would not accept anything that didn't feel like that." People must have thought she was mad. "They still do. They think I am barking mad and dangerous."

Did Beck never have any idea that her father had raped her? "Have you ever almost remembered the name of a movie star? It's on the tip of your tongue, this annoying something you almost remember. I had that all my life, that feeling that something horrible and dark was just behind the screen of my consciousness. I had horrible insomnia, nightmares and this constant nagging feeling that something would erupt into consciousness that was too horrible to cope with. That experience with the light gave me so much comfort that I was finally able to become really conscious of it."

Beck removed herself from her family and left Utah. She now lives in Arizona. It was many years before she wrote a book, Leaving the Saints, about her childhood sexual abuse and about leaving the religion, which caused a huge amount of controversy in the US. When it was published three years ago, she received death threats, and her family, who had ostracised her, still devote a lot of time to discrediting her. She says they accused her of having had sex with the devil when she was a child. "That's their explanation," she says. "Although a lot of them supported me before they knew I was going to tell anyone about it."

She wrote the book, she says, "because I felt compelled to do it, mainly because I knew there were a lot of other people who had been through it. Every friend I had who was raised by one of those old polygamous families, they were all incest survivors. It's very common, and a result of the weird isolation [of the religion]. And my father's work was the bastion, the supporting scholarly work for it."

Shortly after she left Utah, she separated from her husband, who had come out as gay. "He told me, before we even really started dating, that he thought he was gay," says Beck. When they left the church, he told her he felt he needed to be in a relationship with a man. Beck was devastated, but says she had prepared herself for it during their 10 years of marriage. "We talked about getting divorced, but it wasn't like he wanted to marry another woman and I didn't want another man in my life, so we thought we would co-parent the children, live together, be best friends and not have a sexual relationship. But I felt it was very lopsided - what was I supposed to do? The thought of dating other men felt weird."

Instead, Beck found herself becoming drawn to a female colleague, a sociology professor. "One day I suddenly had a slideshow playing in my mind of images of my future, and this woman was in all of them. It literally took the breath out of my lungs." She has been with her partner, Karen, for 14 years. "I had never sexually identified as gay and it was totally unexpected. I think the fact that my husband was gay had opened me to my relationship with Karen - I can't imagine anything else that would have made me do that."

How did her children react? "They were all young. My eldest was eight, Adam was six, and my youngest was four. They assumed it was normal, because we assumed it was normal. Well, I thought it was as strange as could be, but I had just said goodbye to every relationship of my life. It was like my life had burned down and I could rebuild it any way I liked."

· Steering by Starlight, by Martha Beck (Piatkus, £12.99)