Princess Diana's honeymoon letter reveals a young bride 'playing with grown-ups' in the royal family

Princess Diana believed she was headed for a fairy-tale future with then-Prince Charles — until her dream unraveled.

Katherine Hanbury, who attended West Heath Girls' School with the late Princess of Wales between 1973 and 1977, is partnering with U.K.-based auction house Gorringe's to auction photographs and letters beginning July 7.

"She was completely hopeful," the royal's childhood friend told Fox News Digital. "She thought she'd met her prince. And we all hope, don't we, when we get married, that we've found somebody we're going to spend the rest of our lives with, have children with and have a lovely, happy life."

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"She wanted to build a family of her own," Hanbury reflected. "She had all of those aspirations. She was such a lovely, caring girl. And that was definitely what she wanted more than anything else: to have a family of her own, to be loved and cared for, as we all do, really."

"I've had these letters and photos in my desk for the last 45 years or so," Hanbury said. "I'm not getting any younger, and I can't take them with me. I thought now is the time to give somebody else the opportunity to have the pleasure and the joy of looking at and reading them, as they gave to me."

At the girls' school, they became fast friends. Diana was described as "good fun" and "very ordinary" — meant as a compliment.

"My memory of her was that she really enjoyed all the everyday girl things," Hanbury said. "She was very giggly, very caring. That's my memory of her: a young girl who was absolutely delightful. She was just Diana, another girl at the school in our group. She was actually Lady Diana Spencer, but she just went by Diana."

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"She loved all the domestic things," Hanbury said with a laugh. "I remember our headmistress had a small flat within the school, and she asked whether she could go in and clean it for her. She loved tidying people's rooms, organizing drawers and lockers. She loved the domestic side of life. And she was sporty, too. She was good at swimming and loved tennis."

"I remember us riding our bikes down a hill," she continued. "We rode the whole way, just shrieking and hollering, making an awful lot of noise while pedaling as fast as we could. She just loved doing things with you. One of our simple pleasures was riding our bikes and screeching all the way down. She was just a girl like any of us."

Diana left West Heath at 16. She remained in touch with Hanbury.

Charles and Diana didn't begin dating until 1980. The couple married on July 29, 1981.

"I remember her going home for the weekend and coming back so excited," Hanbury said. "Prince Charles had been at Althorp, her parents' country estate. Prince Charles was a friend of her two older sisters, and he happened to be there that weekend when she came home. So when she came back, she was thrilled because she had met the prince. Then the romance was revealed in the papers."

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It was through the press that Hanbury discovered her "ordinary" friend was engaged to the future king of England.

"She was so young; she was only 20," Hanbury said. "She seemed barely out of school. It was portrayed as very romantic. She was definitely smitten with him. I'm sure when she was at school with us, she never thought that one day she would marry a prince. We were all surprised."

The lot includes "The Honeymoon Letter," which the newly minted Princess of Wales wrote to her friend.

"We had a blissful honeymoon with endless sun and luckily calm seas," Diana wrote. "We are now up in Scotland until the end of October, which is a big treat for us — I adore being outside all day & hate London! Its [sic] wonderful being married — I think its [sic] safe to say that after two months...!"

In the letter, Diana hinted at what it was like adapting to her new life as a member of the royal family, writing, "Its [sic] a case of playing with grown-ups!"

"You have to remember she was so young," Hanbury said. "It was all a new playground for her. I imagine she was thinking, 'I'm married, and now I'm a grown-up. I need to start my family.'"

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Hanbury believed Diana's future as a royal was set.

"She was always caring for people," Hanbury said. "She was very thoughtful, kind and loving. She wanted to be loved by everybody. She was a giver. There was no other side to her. She was genuinely kind. When she entered this whole new world — the royal family — everything we saw of her was real. That was Diana. She never lost that, even when her life changed suddenly. That's who she was."

The two women took dramatically different paths. While Diana was thrust onto the world stage, Hanbury continued her education and quietly built a life of her own. Even from afar, she never stopped thinking about her old friend.

"I wondered what it would be like when Charles eventually became our king of England," said Hanbury. "There was always a lot of hope for Diana."

Charles and Diana's marriage was marked by years of turmoil under relentless public scrutiny. After welcoming two sons, Prince William and Prince Harry, the couple announced their separation in 1992. Their divorce was finalized in 1996.

Less than a year later, on Aug. 31, 1997, Diana died after suffering injuries in a car crash in Paris. She was 36.

"You could smell the flowers in London from miles away before you even got to where they were being laid in front of the palace gates," Hanbury said quietly. "It was unbelievably sad. I still can't put it into words. She was so young, and then to die in such a way after enduring all the awfulness that had come before, with her marriage falling apart. It was an absolute tragedy."

"I felt immense sadness," she said. "But millions of people around the world felt the same way, which was remarkable. She was so loved, always. I don't think there will ever be anybody like her."

Today, Hanbury hopes her letters and photographs will introduce the public to another side of the girl she called a friend.

"She was the real deal," Hanbury said. "She never tried to be anyone or anything other than her real self. I think that's something that's missing in a lot of the royal family today, probably. But that's who she was. She was Diana."