Princess Diana staged a confrontation with her husband's longtime lover, Camilla Parker Bowles, saying she knew about the affair with Prince Charles, according to audio tapes aired on Thursday.
The disclosure came in a second round of secretly-recorded tapes, broadcast for the first time in the United States.
In the recordings, made at Kensington Palace in the early 1990s, she also told of her plan to "cut a very different path" from the rest of the Royal Family.
And she said a doctor told her that her bulimia and depression could easily be cured, saying to her: "The problem lies with your husband."
Describing the confrontation with Parker Bowles, which occurred at a birthday party for Ms Parker Bowles' sister in 1989, Diana said: "The voice inside me had said 'just go for it'.
"I was terrified of her. I said, 'I know what's going on between you and Charles and I just want you to know that'."
Diana chose to confront Parker Bowles when she was speaking to the Prince of Wales and another guest.
She said on the tape: "I said to the two men, 'OK, boys, I'm just going to have a quick word with Camilla and 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 is she going to do?'."
She said Parker Bowles told her: "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 do you want?"
"So I said, 'I want my husband'," Diana said.
"And I said to Camilla, 'I'm sorry I'm in the way ... and it must be hell for both of you. But I do know what's going on. Don't treat me like an idiot'."
Never-seen-before video footage was also shown on the programme, aired by the US network NBC.
The film was taken by a voice coach when Diana was training for a new, more public, role as a single woman.
During the session, Princes William and Harry, then 11 and nine, came into the room, although they were not seen on tape.
William asked: "Can I watch this?" Diana answered: "You come in here, sit down and watch this." Gesturing to the video camera she said: "Don't touch it, William, because it's all focused on me."
Later, when Harry laughed, she said: "Harry, shush. Just sit."
William echoed: "Exactly Harry, shush." The coach rehearsed a mock interview with Diana.
Asked why she did charity work, she joked: "Because I've got nothing else to do." William was heard to say: "That's terrible - it's recording."
In the end, she said: "I've not a clue to that question."
Asked what she would do if she were not the Princess of Wales she said: "I would like to have a role which brought me into contact with every day life and the man on the street - does that sound condescending?"
As she grew tired, she said she was "knackered" and could not go on with the session.
Most of the tapes were recorded secretly by Diana and smuggled out of Kensington palace by a friend, Dr James Colthurst.
With Diana's approval, he passed them to Andrew Morton to form the basis of his bestseller, Diana: Her True Story.