Princess Diana knew that Prince Charles loved another woman before their royal wedding.

In the dramatic TV documentary titled "Diana: In Her Own Words," the late Princess of Wales hinted about her knowledge of Prince Charles' love for Camilla Parker Bowles and how the latter broke her heart. 

Princess Diana admitted that she was deeply hurt when Prince Charles left for a five-week royal tour months before their royal wedding. However, the pain was not due to being separated from the prince, but due to Camilla.

"You may recall seeing a picture of me sobbing in a red coat when he went off on his aeroplane," Princess Diana said. "That had nothing to do with him going. The most awful thing had happened before he went. I was in his study talking to him about his trip. The telephone rang, it was Camilla."

According to Princess Diana, she chose to be nice by leaving them talking on the phone. But on the inside, she was deeply hurt. "And it just broke my heart, that," she said.

Even before Prince Charles and Camilla's affair, Princess Diana had already sensed that her groom-to-be was in love with somebody else. It took time before she figured out that it was his ex, Camilla, that he was in love with.

"Instinct in a female - you knew what was coming," Princess Diana said. "I sat up all night with my girls, saying: 'What do I do? What do I say? By that time I had realised there was someone else around."

Princess Diana realized that it was Camilla when she spent a lot of time with her family. She noticed the Duchess of Cornwall's behavior towards Prince Charles and came up with the conclusion that something was going on between the two.

"I couldn't understand why Camilla kept saying to me: 'Don't do this. Don't do that," Princess Diana recalled. "She knew so much about what he was doing privately. Eventually, I worked it all out."

When Prince Charles proposed, Prince Diana thought that it was a joke. But she soon realized that the heir to the throne was serious.

"In my immaturity, which was enormous, I thought he was very much in love with me, which he was," the People's Princess said.

In the same documentary, Princess Diana revealed that Prince Philip advised Prince Charles to get back with Camilla if their marriage doesn't work out after five years. She had a hint about it and only confirmed it when it happened.

According to Sally Bedell Smith's biography "Prince Charles: The Passions and Paradoxes of an Improbable Life," Prince Charles has always loved Camilla. Unfortunately, the royals perceived her as an "experienced" woman. They also didn't find her aristocratic enough for the future king so they preferred Princess Diana.