By 1989, Charles and Diana were leading largely separate lives, the Prince having rekindled his affair with Camilla Parker Bowles some years before. Enraged by the depth of his attachment to Camilla, Diana confronted her at a party.
As she told royal author Andrew Morton two years later, in tape recordings she made secretly, it was one of the bravest moments of her life.
Now, 20 years after her death, the book he wrote - Diana, My True Story - is being republished, with transcripts of the tapes, according to the Daily Mail.
The final extract begins with Diana's account of that confrontation - and the relief she felt afterwards.
The worst day of my life was realising that Charles had gone back to Camilla. One of the bravest moments of my ten years of marriage was when we went to this ghastly party (thrown by Lady Annabel Goldsmith in February 1989) for Camilla's sister's 40th birthday.
Nobody expected me to turn up, but a voice inside me said: "Go for the hell of it." So I psyched myself up something awful.
I decided I'm not going to kiss Camilla hello anymore. I was going to shake hands with her instead. This was my big step.
And I was feeling frightfully brave and bold, and basically: "Diana's going to come away having done her bit."
Charles needled me the whole way to Ham Common (in Richmond, South-West London), where the party was.
"Oh, why are you coming tonight?" - needle, needle, needle, the whole way down.
I didn't bite but I was very, very on edge.
Anyway, I walk into the house and stick my hand out to Camilla for the first time and think: "Phew, I've got over that."
There were about 40 of us there and we all sat down and, bearing in mind they were all my husband's age, I was a total fish out of water. But I decided I am going to try my hardest. I was going to make an impact.
And then after dinner, we were all upstairs and I was chatting away, and I suddenly noticed there was no Camilla and no Charles. So this disturbed me.
So I make my way to go downstairs. I know what I'm going to confront myself with.
They tried to stop me going. "Oh, Diana, don't go down there."
"I'm just going to find my husband - I would like to see him."
I had been upstairs about an hour and a half so I was entitled to go down and find him. I go downstairs, and there is a very happy little threesome going on - Camilla, Charles and another man chatting away.
So I thought: "Right, this is your moment," and joined in the conversation as if we were all best friends. And the other man said: "I think we ought to go upstairs now."
So we stood up, and I said: "Camilla, I'd love to have a word with you, if it's possible," and she looked really uncomfortable and put her head down.
And I said to the men: "OK, boys, I'm just going to have a quick word with Camilla - I'll be up in a minute," and they shot upstairs like chickens with no heads, and I could feel upstairs all hell breaking loose. "What's she going to do?"
I said to Camilla: "Would you like to sit down?" So we sat down, and I was utterly terrified of her, and I said: "Camilla, I would just like you to know that I know exactly what is going on."
She said: "I don't know what you're talking about!"
And I said: "I know what's going on between you and Charles, and I just want you to know that."
And she said: "Oh, it's not a cloak-and-dagger situation."
I said: "I think it is."
I wasn't as strong as I'd have liked, but at least I got the conversation going.
She told me: "You never let him see the children when he's up in Scotland."
I told her: "Camilla, the children are either at Highgrove or in London." That's Charles' biggest fault: he never sees the children. But I never take them away.
The other day, for instance, William said: "Papa, will you play with us?" "Oh, I don't know if I have time." Always happens. So, he can't gripe about that.
Anyway, going back to Camilla. She said to me: "You've got everything you ever wanted. You've got all the men in the world falling in love with you, and you've got two beautiful children. What more could you want?"
So I said: "I want my husband."
Someone came down to relieve us, obviously. "For God's sake, go down there, they're having a fight." It wasn't a fight - calm, deathly calm.
I said to Camilla: "I'm sorry I'm in the way. It must be hell for both of you, but I do know what is going on. Don't treat me like an idiot." So I went upstairs and people began to disperse.
In the car on the way back, my husband was over me like a bad rash and I cried like I have never cried before - it was anger, it was seven years' pent-up anger coming out.
I cried and cried and cried and I didn't sleep that night. And the next morning, when I woke up, I felt a tremendous shift. I'd done something, said what I felt. Still the old jealousy and anger swilling around, but it wasn't so deathly as before.
And I said to him at the weekend, three days later: "Darling, I'm sure you'll want to know what I said to Camilla. There's no secret. You may ask her. I just said I loved you - there's nothing wrong in that."
He said: "I don't believe it."
I said: "That's what I said to her. I've got nothing to hide - I'm your wife and the mother of your children."
That always makes him slightly twitch, when I say "mother of your children". He hates being made aware of it.
That was it, really. It was a big step for me. I was desperate to know what she said to him - no idea of course!
My shrink said: 'It's him, not you'
My husband made me feel so inadequate in every possible way: each time I came up for air, he pushed me down again.
Even if I ate a lot of dinner, Charles would say: "Is that going to reappear later? What a waste."
I think the bulimia woke me up (in the end). I suddenly realised what I was going to lose - and was it worth it? (My former flatmate) Carolyn Bartholomew rang me up one night and said: "Do you realise that if you sick up potassium and magnesium, you get these hideous depressions?"
I said: "No."
"Well, presumably that's what you suffer from. Have you told anyone?" I said: "No."
"You must tell a doctor."
She said: "You must. I'll give you one hour to ring up your doctor - and if you don't, I'm going to tell the world." She was so angry with me. So that's how I got involved with the shrink Maurice Lipsedge. He came along - a sweetheart, very nice.
He walked in and said: "How many times have you tried to do yourself in?" I thought: "I don't believe this question," so I heard myself say: "Four or five times."
He asked all these questions, and I was able to be completely honest with him. I spent a couple of hours with him, and he said: "I'm going to come and see you once a week for an hour and we're just going to talk it through."
He said: "There's nothing wrong with you; it's your husband."
And when he said that, I thought: "Maybe it's not me."
He helped me get back my self-esteem, and he gave me books (on bulimia) to read. I kept thinking: "This is me, this is me - I'm not the only person!"
Dr Lipsedge said: "In six months' time, you won't recognise yourself. If you can keep your food down, you will change completely."
I must say, it's like being born again since then. Just odd bursts, lots of odd bursts - especially at Balmoral, very bad at Balmoral. And Sandringham and Windsor. Sick the whole time.
Last year, I was all right: it was once every three weeks, whereas it used to be four times a day. And it was a big "hooray" on my part. I felt so much stronger mentally and physically, so was able to soldier on in the world.
My skin never suffered (from the bulimia) nor my teeth. When you think of all the acid! I was amazed at my hair.
I thought because the public saw a smiling picture on the front of the Daily Mail, they'd think I was all right. I guess they did wonder, but nobody voiced it.
The dressmakers noticed - but it's like doctors who say: "Oh you've lost a bit" or "What's happened here? You must look after yourself." But that was the extent anyone ever went into it.
I hated myself so much. I didn't think I was good enough. I wasn't good enough for Charles, I wasn't a good enough mother - I mean, doubts as long as one's leg.
I've got what my mother's got. However bloody you're feeling, you can put on the most amazing show of happiness.
My mother is an expert at that. Because I had a smile on my face, everybody thought I was having a wonderful time. That's what they chose to think - it made them happier.
I always knew I would never be Queen
From day one, I always knew I would never be the next Queen. No one said it to me; I just knew.
I got an astrologist in - Fergie introduced me to Penny Thornton. I said: "I've got to get out. I can't bear it any longer."
And she said to me: "One day, you will be allowed out, but you will be allowed out as opposed to divorcing or something like that."
It always sat in my mind. She told me that in 1984, so I've known it for some time.
My grandmother (the late Countess Spencer, Diana's paternal grandmother) looks after me in the spirit world. I know that for a fact.
She used to stay at Park House (Diana's childhood home in Norfolk) with us. She was sweet and wonderful and special. Divine, really.
(When I saw a clairvoyant) my grandmother came in first, very strong, then my uncle and then Barry (Mannakee, her former police protection officer, who died in a motorcycle accident in 1987).
I hesitated about asking questions about Barry because - well, I don't know - I just hesitated. But I've always had a question mark about his death, and I've been given an answer, and that's the end of that.
I've got a lot of déjà vu. Places I think I've been before, people I've met. I've had various rooms around the world where I feel I've been before. One just feels: "Goodness, oh!" But I don't go and talk to anyone about it. I'm just aware that it's déjà vu; it's very strong.
I have an awful lot of dreams about things, if someone's troubled. I knew Adrian (Ward-Jackson, a friend suffering from Aids) wasn't very well this weekend, and I couldn't get through.
But something told me something was wrong, and he had a very, very bad weekend.
I recall sitting in a Land Rover with my policeman and watching (Prince Charles's) horse (Alibar) coming along and rearing its head back. I said: "That horse is going to have a heart attack and die." And it did. It had a heart attack then and there.
I had a premonition (in 1978) that Daddy was going to be ill while I was staying with some friends in Norfolk. They said: "How's your father?" and I said: "I've got this strange feeling that he's going to drop down, and if he dies, he'll die immediately; otherwise he'll survive."
I heard myself say this - thought nothing more about it. Next day, the telephone rang and I said to the lady, "That will be about Daddy." It was.
He'd collapsed. I was frightfully calm, went back up to London, went to the hospital, saw Daddy was gravely ill.
Time is precious... I've got lots to do
If I was able to write my own script, I'd say that I would hope that my husband would go off, go away with his lady and sort that out and leave me and the children to carry the Wales name through to the time William ascends the throne. And I'd be behind them all the way.
I can do this job so much better on my own; I don't feel trapped . . . It would be quite nice to go and do things like a weekend in Paris, but it's not for me at the moment.
But I know, one day, if I play the rules of life - the game of life - I will be able to have those things I've always pined for, and they will be that much more special because I will be that much older and I'll be able to appreciate them that much more.
I don't want my friends to be hurt and think I've dropped them, but I haven't got time to sit and gossip. I've got things to do and time is precious.
Last August, a friend said to me that I'm going to marry somebody who's foreign, or who has got a lot of foreign blood in them. I thought it was interesting.
I do know I'm going to remarry or live with someone.
I see myself one day living abroad. I don't know why I think that - and I think of either Italy or France, which is rather unnerving. But not yet.
Extracted from Diana: Her True Story - In Her Own Words by Andrew Morton.