Prince Charles and Princess Diana
Prince Charles wrote a note to Princess Diana on the night before their royal wedding. Prince Charles and Princess Diana at Westminster Abbey, London, for a centenary service for the Royal College Of Music on Feb. 28, 1982. Getty Images/Fox Photos

Prince Charles thought that marrying Princess Diana would resolve some of their issues.

According to royal author Penny Junor, in her book "The Duchess: The Untold Story" the Prince of Wales penned a private letter to Princess Diana on the night before their wedding. In the letter, Prince Charles revealed how proud he was for his bride-to-be.

"The night before the wedding, which Diana spent at Clarence House with her sister Jane, he sent her a note, along with a signet ring that bore the Prince of Wales feathers," Junor wrote (via Express). "I'm so proud of you and when you come up I'll be there at the altar for you tomorrow. Just look 'em in the eye and knock 'em dead."

Junor noted that Prince Charles initially feared marrying Princess Diana would be a wrong decision after she changed in the run-up of their royal wedding. The People's Princess lost weight and became moody and tearful. However, Prince Charles hoped that their wedding would solve it. Apparently, it didn't.

"Charles was not convinced he was doing the right thing in marrying Diana but there was no way out and, bolstered by the hope that things would be different once they were married, he put a brave face on it," Junor added.

According to royal expert Katie Nicholl, in her book "The Making of a Royal Romance," Princess Diana nearly called off her wedding to Prince Charles because she felt "cut off" and "isolated." In addition, they had different expectations.

Junor agreed with this. According to her, Princess Diana "lived in a romantic world" which clouded her judgment of the realities. She was under the impression that her royal life would be "something out of a romantic novel."

On their wedding day, both showed signs of a failed marriage. Their actions could be easily perceived as "signs of shyness and nerves prompted by love," but it's actually not.

For instance, Judi James noticed Prince Charles' "steepled brows" while waiting for his bride which showed an expression of "duty" and he reportedly looked "like one quiet suffering."

Dr. Peter Collett also saw signs of unhappiness. At one point, Prince Charles wiped away his imaginary tears that suggest that he was having deep regrets about what he has got himself into.