Prince Charles once asked Princess Diana why she was always so "miserable" - and told her to be more fun like Sarah Ferguson.
According to royal author Andrew Morton, Diana then donned leather trousers at a David Bowie concern to try and appear more laid back.
His book Diana: Her True Story — In Her Own Words marks 25 years since her tragic death.
Mr Morton claims the Princess felt so upset she attempted to copy the Duchess of York - but quickly regretted trying to make him happy.
Diana also spoke of the collapse of their marriage, opening up on a face-to-face confrontation with Camilla Parker Bowles.
In one explosive chapter, she detailed the misery of her relationship and her struggle with bulimia.
"My husband made me feel so inadequate in every possible way: each time I came up for air, he pushed me down again," she said in extracts of the book being serialised by the Daily Mail.
"Fergie seemed to know all about the royal set-up, and came to lunch at Buckingham Palace and didn’t seem daunted by it all
"Suddenly, everybody said: ‘Oh, isn’t Fergie marvellous, a breath of fresh air — thank God she’s more fun than Diana.’
"I felt terribly insecure. I thought maybe I ought to be like Fergie.
"And my husband said: ‘I wish you would be like Fergie — all jolly. Why are you always so miserable?
"Why can’t you be like Grannie [the Queen Mother]?’ I’m quite glad I’m not like Grannie now. And I made so many balls-ups trying to be like Fergie."
She added: "Fergie was a different kettle of fish altogether, and she wooed everybody in this family and did it so well. She left me looking like dirt.
"I knew eventually Fergie would turn round and say: ‘Duch [Diana’s childhood nickname], how on earth have you survived all these years?’ She’s said it now for the past two years."
Royal commentator Kristen Meinzer backed up that Fergie was a Firm favourite.
She said: "I think a lot of the world forgets that for a while there, Fergie was wildly popular, popular beyond Princess Di.
"So popular that Prince Charles even said at one point to Princess Diana, 'why can't you be a little bit more like her'.
"She's so bubbly, she's so effervescent."
When Morton published his biography Diana in 1992, it was condemned by the Archbishop of Canterbury.
He faced so much criticism one MP suggested he should be banished to the Tower of London.
But he defends his writing as Diana's truth, and while he dismissed conspiracy theories surrounding her death, he adds, she was "absolutely" being conspired against generally.
