In the documentary, which features audio footage of the princess that she secretly had recorded for her biography, she called her wedding day the "worst day of my life."
She also said she confided in her sisters before the wedding, who told her: "Your face is on the tea towels so you're too late to chicken out."
The princess spoke candidly about the royal family, mental health, and the breakdown of her relationship with the Prince of Wales in a series of secret tapes recorded in 1991. Diana gave a friend permission to record her speaking on behalf of the journalist Andrew Morton for a book about her life, "Diana: Her True Story."
It was not revealed that Diana was involved in the biography until after her death, when Morton released a revised version of the book. Then, in 2017 National Geographic featured the tapes in the documentary, "Diana: In Her Own Words," which has since been made available on Netflix and Disney Plus.
Speaking in the audio tapes, Diana said of her wedding day: "I don't think I was happy. I never tried to call it off, in the sense of really doing that, but I think [it was] the worst day of my life."
She also described herself as being "a lamb to the slaughter."
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The princess explained in the tapes how she had been feeling doubts about her relationship with Charles in the lead up to the royal wedding.
She said when they were engaged she discovered a bracelet that the prince had engraved especially for Camilla, at which point she told her sisters she didn't want to marry him.
"Bad luck Duch," they responded, according to Diana. "Your face is on the tea towels so you're too late to chicken out."
Diana and Charles tied the knot at St Paul's Cathedral in London on July 29, 1981. The ceremony was watched by a global TV audience of 750 million people, according to the BBC.
They were the first royal couple to share their first kiss in front of the public on the balcony at Buckingham Palace, a tradition that has since been followed by Prince William and Kate Middleton in 2011, and Prince Harry and Meghan Markle outside St George's Chapel in Windsor in 2018.
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Diana and Charles separated in 1992 and divorced in 1996. The prince went on to marry Camilla in 2005, and she was given the title Duchess of Cornwall.
The Prince and Princess of Wales' relationship has been the focus of season four of "The Crown" which aired on Netflix earlier this month. The historical drama portrays the beginning of Charles and Diana's marriage and the prince's affair with Camilla Parker-Bowles.
The portrayal of the love triangle in the series has led Diana fans to troll Charles and Camilla's shared social media accounts. In one Instagram post of the Duchess of Cornwall sharing support for the Queen's Commonwealth Essay Competition, fans flooded the comments section with Diana's name.
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