Author Andrew Morton was one of the few people who knew about Diana's struggles as she agreed to record her thoughts onto tapes via a go-between. In 1992, he published a book, Diana: Her True Story - In Her Own Words, which became a sensation.
According to the Daily Mail, now 20 years after her death, the book is being republished, with transcripts of those tapes.
The pair met when Diana was just 16. Charles, 29, was then dating her older sister Sarah. After meeting Diana at his 30th birthday at Buckingham Palace the pair got married in 1981. But their marriage was headed to a certain doom and according to the tapes Diana was aware of Charles and Camilla Parker Bowles' romance even before they were married.
According to the Daily Mail report, Diana claimed in the tapes: "The first thing that hit me was my (future) husband sending Camilla Parker Bowles flowers when she had meningitis: 'To Gladys from Fred' (their nicknames for each other). I never dealt with that side of things. I just said to him: 'You must always be honest with me.'"
After their royal engagement, Diana overheard Charles talking to Camilla. She details via the tapes, "I was in his study talking to him, when the telephone rang. It was Camilla. I thought: 'Shall I be nice (and leave him alone so he can talk to her in private) or shall I just sit here?' So I thought I'd be nice, so I left them to it. It just broke my heart, that."
Detailing more about Charles and Camilla's affair, Diana is quoted as saying, "We always had discussions about Camilla, though. I once heard him on the telephone in his bath on his hand-held set, saying: 'Whatever happens, I will always love you.' I told him afterwards that I had listened at the door, and we had a filthy row."
Diana discovered Charles had a gold chain bracelet with a blue enamel disc made for Camilla. The bracelet had 'G and F' entwined in it with their initials of their nicknames for each other- Gladys' and 'Fred'.
The tapes also revealed how the princess began losing weight even before her wedding day when she was still a teenager."The bulimia started the week after we got engaged," Diana said.
"My husband put his hand on my waistline and said: 'Oh, a bit chubby here, aren't we?' and that triggered off something in me. And the Camilla thing. I was desperate, desperate. I remember the first time I made myself sick. I was so thrilled because I thought this was the release of tension."
She also shared that the first time she was measured for her wedding dress, she was 29 inches around the waist and on their marriage day, she was just 23½ inches. "I had shrunk into nothing from February to July. I had shrunk to nothing," she said.
The couple's divorce was finalised in August 1996, 11-year after their marriage. Diana passed away at the age of 36 in a tragic accident in Paris in 1997.