The insider added: “Perhaps because it was a very difficult time for her, but she felt them slipping away and it upset her. She knew in a few short years, they’d be young men with their own lives and she feared since she’d split from the royal family, they would freeze her out.” But, as had been the case for most of her life, Diana was able to hide her heartbreak behind a breathtakingly glamorous exterior. On July 1, 1997, she celebrated her 36th birthday as the guest of honor at the 100th anniversary of the Tate Gallery in London. Her brother, Charles Spencer, accompanied her. Spencer recalled the last night he spent with his sister two months later in the heartbreaking eulogy he delivered at her funeral. “The last time I saw Diana was on July 1, her birthday in London, when typically she was not taking time to celebrate her special day with friends but was guest of honor at a special charity fundraising evening,” he said. “She sparkled, of course.” The night of the gala, Diana dazzled the crowd when she emerged from her car in one of the most daring gowns she’d ever worn. Her black beaded lace gown by Jacques Azagury was a birthday gift from the designer and decidedly more low cut than anything she’d ever worn as a member of the royal family. She accessorized her look with a stunning emerald and diamond choker and matching drop earrings from her jewelry collection. Azagury designed the gown to mirror a shorter dress in powder blue he made for the princess that she’d worn to the London Ballet a month prior. In speaking to Azagury for my book Diana: The Secrets of Her Style, he said that, in the wake of her divorce from Prince Charles, Diana was excited about wearing sexier, more body conscious clothes that showed off her figure. During the fitting, he recounted that he told her, “It’s a little too low.” But Diana’s eyes brightened as she said, “‘No, it looks great,’” recalled Azagury. “She had a good eye and knew immediately what she wanted. She came to me for a specific kind of dress,” he said. But Diana’s dazzling appearance that night belied a truly lonely period in her life. As the one-year anniversary of her divorce from Charles neared, Dr. Hasnat Khan, the Pakistani heart surgeon she was in love with, was beginning to pull away and William and Harry would soon be leaving London to spend August in Balmoral with the royal family. (Khan would break up with Diana a week later.) “The boys loved being in Scotland doing all the things Diana detested—killing things and mucking around,” said my source. “There was even a go-cart track on the estate that William and Harry loved. They were quite limited when they were at Kensington Palace with Diana. She knew as they got older, it was no longer very much fun for them to be in London with her.“ae0fcc31ae342fd3a1346ebb1f342fcb RELATED: For more up-to-date information, sign up for our daily newsletter. But the boys were fiercely devoted to their mother, even if their teenage pursuits often turned their interests elsewhere. At 15, William was his mother’s trusted confidant. He had given her the idea for the highly successful auction of her dresses at Christie’s that raised millions of dollars for AIDS and cancer research. She had shared the details of her relationships with him and even discussed the terms of her divorce with her son before finalizing the agreement. In a 1997 interview for The New Yorker, Diana told Tina Brown, “All my hopes are on William now.” Both William and Diana worried about Harry and frequently talked about their concern for him over feeling overshadowed and overlooked in his role as “the spare.” On her last birthday, before she headed out to the Tate, she received 90 bouquets of flowers from close friends and the charities she supported. Harry called his mother from school and enlisted the aid of his classmates to sing “Happy Birthday” to her. “But when she returned to Kensington Palace that night, she was alone,” said my source. “She was resolute about finding a way to have a wonderful time with her sons that summer. No one could have predicted how wrong it would all ago and that would be the last time they would be together.” And for more on that fateful summer, check out The 6 Biggest Unanswered Questions Surrounding Princess Diana’s Death. Diane Clehane is a New York-based journalist and author of Imagining Diana and Diana: The Secrets of Her Style.