Royal watchers remember Princess Diana through her signature smiles, but behind those beams was a broken heart.

In the 1981 interview by The Telegraph, Prince Charles reminisced how she saw Princess Diana when she was still a teen.

"I remember thinking what a very jolly and amusing and attractive 16-year-old she was," the Prince of Wales stated. "I mean, great fun, and bouncy and full of life and everything."

However, much like their short-lived marriage, the amusement that he felt only lasted for a short time.

Marriage Bad For Diana's Mental Health

The sunny-looking and lighthearted Princess of Wales turned into a person who struggled with mental health issues alone. Prince Charles, instead of helping her out, made everything even worse.

The documentary entitled "Diana: In Her Own Words" revealed what happened during the interview of Princess Diana with her friend James Colthurst at London's Kensington Palace in 1991.

Princess Diana's brutally honest stories focused not only on how Prince Charles became unfaithful to her but also how he spoke impertinent comments that caused her eating disorder.

According to the late royal princess, her bulimia battle started even before she got engaged with Prince Charles. It began when the heir to the throne held her waistline and said, "Oh, a bit chubby here, aren't we?" -- prompting something unpleasant inside her.

Moreover, a few months after their 1981 Wedding of the Century, Diana's suicidal thoughts got even worse to the point that the royal family sought for the experts to "fix her."

"I was about to cut my wrists. I came [back to London] to seek treatment. I was in such a bad way. Couldn't sleep, didn't eat, the whole world was collapsing around me," Princess Diana revealed.

Diana added that analysts and psychiatrists gave her high doses of valium, a medication to calm her down, in pursuit of "sorting her out."

Even Princess Diana's former head chef Darren McGrady noticed that something was not right about the royal princess.

"I have never talked about the food I prepared for her, the silly things she'd ask for, and I'd never want to and never will, I don't think it's right," McGrady said.

Princess Diana Kept a Secret to Not Disappoint Prince Charles

In September 1984, the birth of Prince Harry shocked Prince Charles the most, not because he did not expect it at all but because he expected a girl.

The interesting story made it to Andrew Morton's book entitled "Diana: Her True Story." Instead of celebrating the baby's birth, people heard Prince Charles said "Oh God, it's a boy" first before commenting about the little prince's red hair.

According to a royal biographer, Prince Charles wanted to have a daughter. He was excited to have a second baby in hopes it would be a girl, only to find out that Princess Diana knew that the baby was a boy all along through a scan.

Diana, however, chose not to tell the Prince of Wales about Harry's gender since she did not want to disappoint her husband.

To his disappointment, Prince Charles vented out to Princess Diana's mother, Frances Kydd, during Harry's christening. Even months after, he whined about not having a daughter. However, Kydd made the prince realize how lucky he was to have a normal child.

READ MORE: Royal Trauma: Why Princess Diana Thought Marrying Prince Charles 'Was Too Abrupt'