Unable to continue his relationship with Camilla Parker-Bowles, since she was already married, he was heartbroken marrying someone else.
The loss of a loved one through any means is a pain that can stay in a person's heart for many moons. Prince Charles was so heartbroken over losing the woman he truly loved, that he wept the night before he was supposed to marry Lady Diana, who went on to become the people's princess but not one of his heart.
Charles met Camilla Shand (later Parker-Bowles) in the 1970s and had a short fling. This casual relationship later blossomed into a serious one while she was married to Andrew Parker-Bowles. Their chemistry was instant and electric right from the time they met, each other and even while Charles fell deeper in love with her, she was conflicted. He loved that she smiled with her eyes, and they shared a laugh about the same silly things. He also loved how natural and friendly she was with him instead of fawning over him.
Soon after they met, he started calling her to talk more and know her better. Even though he was busy, he made time for her. 22-year-old Charles was in the middle of intensive military training after qualifying as a jet pilot with the Royal Air Force, according to Daily Mail.
It wasn't until late autumn of 1972 that they started seeing each other. Both of them shared a love for and played polo at the Guards Polo Club in Windsor Great Park. She had also been a regular there, as she would go to watch Parker-Bowles play. Even though she was part of the most elite clubs in London, she wasn't "sufficiently aristocratic" for Lord Mountbatten to approve of her. His approval was important to Charles and the Crown.
Mountbatten, who was Prince Philip's uncle, is believed to have said "lovely for you two to have a fling, but this absolutely cannot end in marriage" to Charles, as per Express UK.
Eventually, in 1973, she made up her mind to marry Parker-Bowles. "Nothing was going to stop her," writes Britain’s top royal author Penny Junor. Charles had to go to the West Indies for his service in the Royal Navy in the early part of 1973, and he didn't confess to Camilla the depth of his feelings.
She was flattered by the prince's attention but was still in love with Parker-Bowles, whom she married in 1973 even though he'd had a brief affair with Charles' sister Anne. The cavalry officer had always been with more than one woman at a time even when he was dating Camilla, and even when he was married to her. "She adored him, and she had been dating him through thick and thin for seven years. She wanted to be Mrs Parker Bowles, wife of her handsome cavalry officer, not Princess of Wales, not Queen," writes Junor.
Before she got married, when Charles found out about her decision, he wrote to her beseeching her not to marry the philandering cavalry officer. He wrote in one letter that after "such a blissful, peaceful and mutually happy relationship," it was fate's decree that it ends in six months. "I suppose the feeling of emptiness will pass eventually," he mourned for his relationship that didn't see a proper end.
However, once she had married Parker-Bowles, she experienced more pain as he kept seeing other women, some of them her friends. It was this pain of betrayal that led her back to Charles after her marriage. They had become best friends; their relationship was platonic initially, and Charles was also asked to be her son Tom's godfather. Camilla and her husband were also close to the Queen and the Queen Mother.
Around late 1978, after the birth of her daughter, their feelings for each other were rekindled, and they were in a relationship up until he got engaged to Diana. The night before his wedding, the prince reportedly wept at the window of the Buckingham Palace. The third time they started seeing each other again was around five years after he got married, according to the SUN UK. Things went downhill in his marriage with Diana, who had become aware of his infidelity, and it led to a very public divorce. He eventually married Camilla in 2005, after Diana's death in 1997.
References:
https://www.dailymail.co.uk/news/article-4634144/Camilla-s-story-revealed-explosive-new-book.html
