Prince Charles reportedly sought his father's advice regarding marrying a woman he doesn't love. 

The Duke of Edinburgh was aware that the Prince of Wales loved somebody else before he married Princess Diana. However, Prince Philip encouraged his son to marry Princess Diana anyway as she was a better choice than Camilla Parker Bowles. But he apparently said that if their marriage doesn't work after five years, then he can get back with Camilla.

"My father-in-law said to my husband, 'If your marriage doesn't work out, you can always go back to her after five years,'" Princess Diana said in a 2017 Channel 4 special that utilized taped conversations between the Princess of Wales and her voice coach (via E! News). "Which is exactly—I mean, for real I knew that it had happened after five—I knew something was happening before that but the fifth year I had confirmation."

According to Paul Burrell in his 2003 book "A Royal Duty," the royals are taught to "compromise, be less selfish and try to work through their difficulties for the sake of the monarchy, their children, the country and its people." Unfortunately, Prince Charles couldn't move on from Camilla and Princess Diana would not settle for a marriage of three.

In respect to Princess Diana, Camilla was not called Princess of Wales, although she holds the title. Also, it was announced that she would only be Princess Consort and not Queen Consort.

Prince Charles and Camilla briefly dated before they married their ex-spouses. According to Sally Bedell Smith's biography "Prince Charles: The Passions and Paradoxes of an Improbable Life," the royals didn't approve of Camilla for the heir to the throne.

The family of the Prince of Wales perceived the Duchess of Cornwall as an "experienced" woman which was a nonstarter for them in terms of a suitable wife for the future king. In addition, they didn't find Camilla to be "aristocratic" enough for Prince Charles.

"Lovely for you two to have a fling, but this absolutely cannot end in marriage," Prince Charles' great-uncle, Lord Mountbatten, reportedly advised him.

Despite the opposition of the royals, Prince Charles couldn't get over his love for Camilla. He revealed that they had an affair while he was still married to Princess Diana.

Prince William and Prince Harry's mom said in an interview when she was alive that their marriage was difficult as there was three of them. Her former royal butler also revealed that she suspected Prince Charles of plotting an accident to kill her and marry Camilla.

"Her words were 'these next few months, are the most difficult months of my life. I fear my husband is going to kill me. In an automobile accident. With head injuries. In order that he can remarry,'" Burrell recalled. "That is spooky. She predicted her own death nine months before she died."