Princess Diana candidly discusses marriage issues, battling bulimia and more in secret recordings from 1991

Just weeks ahead of the 20th anniversary of Princess Diana's tragic death, a number of documentaries and television specials have aired in remembrance of the beloved royal -- but only one is in the voice of the princess herself.

In 1991, Princess Diana participated in a series of secret interviews at Kensington Palace that were ultimately used for a book about her life that was written by journalist Andrew Morton and published the following year.

For the first time, those recordings can be heard in National Geographic's new documentary, Diana: In Her Own Words, which airs Monday in the U.S. at 9/8 Central on Nat Geo. The special weaves archival footage and photography of Diana in with her talking about how she came to meet Prince Charles and ultimately become his wife. Her extreme candor does not paint a pretty portrait of life in the palace, as she admits to battling bulimia after getting engaged, having thoughts of suicide and being constantly suspicious of Prince Charles' relationship with Camilla Parker Bowls, who now goes by the Duchess of Cornwall and is married to Prince Charles.

WATCH: Princess Diana Biographer Claims in New Special She Was at 'Her Wit's End' When She Made Secret Recordings

Diana recalls one of the first times she kissed Prince Charles was after he had broken up with another girl and had just attended the funeral of Earl Mountbatten. "We were at a barbecue that night talking about Mountbatten and his girlfriend and I said, 'You must be so lonely,'" she's heard saying in the recordings, noting that she told him, "'You need someone beside you...'"

That's when Prince Charles made his move, according to Diana. "Whereupon he leapt upon me and started kissing me and everything and, you know, this is not what people do."

Diana claims she and the prince only met 13 times before their wedding day, but he was initially "all over [her] like a rash."

"He would ring me up every day and then not contact me for weeks," she remembers. "He wasn't very consistent in his courtship."

After marrying Prince Charles in 1981, Diana says that she started to feel very isolated from the royal family, especially after it became known that she was struggling with an eating disorder. "Everybody knew about the bulimia in the family," she says. "They all blamed the failure of the marriage on the bulimia, and that's taken some time to get them to think differently."

In another big reveal, Diana implies that Prince Charles was more than friends with Camilla even before their engagement. At one point, Diana says she confronted Camilla, telling her, "I'm sorry I'm in the way. 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."

Diana goes on to talk about her role as a royal. "I am performing a duty as the Princess of Wales as my time is allocated," she notes. "If life changes, it changes. ...I know when I turn my light off at night, I did my best. I always knew I'd never be the next queen, put it that way."

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Prince Charles and Diana divorced in 1996. She died on Aug. 31, 1997, after a fatal car accident in Paris, France.