Read Princess Diana's Secret Love Letter to Charles on Her 65th Birthday — Including Shocking Confession About Their Honeymoon

When Princess Diana married the then-Prince Charles, she was only 20 years old, and like any young bride, she hoped she and her new husband would enjoy a long, happy life together.

But when her 36-year-old husband turned out to be a cold fish, OK! can now reveal she didn't give up.

The beautiful royal – who would have toasted her 65th birthday this month had she survived the Paris car smash in 1997 that claimed her life – poured her heart into secret letters addressed to Charles, believing if she wrote the right words – and if she expressed them beautifully enough – she could bridge the distance between them.

But sources tell us for every letter she sent, there was silence in return.

Diana, young and full of hope, penned the tender, longing letters to her husband in the early years of their marriage.

In them, she detailed her dreams for their future, whispered affections onto pages meant to rekindle the love she thought they once shared.

But Charles, preoccupied and emotionally distant, never wrote back.

Former royal butler Paul Burrell said: "It's such a sad thing to say that he never loved her, and so he never returned the compliment. He wasn't romantic. He tried to be, but he didn't have a romantic bone in his body."

The groom did, however, try his best.

The night before his July 29, 1981, wedding to Diana, she received a special letter and a gift from her husband-to-be.

The late royal later said: "He sent me a very nice signet ring the night before to Clarence House, with the Prince of Wales feathers on and a very nice card that said, '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.'"

Though the gift and the letter were touching, and Diana was "so in love" with Charles, their marriage never stood a chance. He was already in love with Camilla Parker Bowles, whom he would marry in 2005, eight years after Diana's tragic death in a Paris car crash.

But the People's Princess was full of hope, which already started to ebb away on her and Charles' honeymoon.

Princess Diana also may have detailed her honeymoon to Charles as a "tremendous success" in a 1981 letter to her family's former housekeeper, but she later confessed the trip was a nightmare.

According to royal author Penny Junor – who wrote the biography The Duchess – Charles packed novels and art supplies for the couple's honeymoon, which saw them visit several destinations, including Tunisia, Algeria, Sicily, the Greek islands, Egypt and Scotland.

Charles had hoped they'd share the books and then discuss them in the evenings, but his new bride didn't want to discuss literature during the honeymoon; she wanted to spend time with him.

"She hated his wretched books and was offended that he might prefer to bury his head in one of them rather than sit and talk to her," said Junor. "She resented (Charles) sitting hours at his easel, too, and they had many blazing rows."

In fact, one day when Charles was painting on the deck of the Britannia, he went off to look for something. He came back to find Diana had smashed his easel and art supplies to smithereens.

They also fought over the fact Charles wore cufflinks Camilla had given him – showing two Cs entwined. Diana asked him, "Camilla gave you those, didn't she?" He said, "Yes, so what's wrong? They're a present from a friend."

Diana later said, "And boy, did we have a row. Jealousy, total jealousy."

Many years later, a royal maid stumbled upon a forgotten bundle of Diana's lovelorn letters tucked inside an old writing desk at Kensington Palace.

The intimate writings lay undiscovered for decades, only coming to light when palace staff moved furniture during a renovation.

The desperate letters are filled with the yearnings of a woman trapped in a loveless marriage.

"She never stopped trying," a royal insider told us. "Diana loved Charles so much, but he never gave her that love in return. It broke her."

The letters, never meant for the public eye, reveal the heart-breaking truth of Diana's marriage – a woman who loved deeply, but was left longing for a love that never truly came.

In the letters, Diana poured out the secrets of her lonely heart. One of them read: "I miss you. Even when you're beside me, I miss you. I wonder, do you ever feel the same? Or is it just me, lost in this love, trying to make sense of what we've become? I think about our wedding day and how I felt walking to-ward you – like I was stepping into a dream, a fairytale come true. But fairytales don't have quiet, lonely nights. They don't have the silence we sit in now."

In another, the tragic princess penned: "Sometimes I watch you when you don't realize it. You look away when I reach for your hand. You nod politely when I tell you I love you, but you never say it back. Do you? Do you love me? Or am I simply someone who walks beside you, plays the part of a princess while you dream of another? If there is even a part of your heart that belongs to me, please, tell me. I will wait for your reply."

The now-King Charles, 77, and Queen Camilla, 78, married on April 9, 2005.

They tied the knot in a private civil ceremony at the Windsor Guildhall, which was then followed by a religious blessing at St George's Chapel in Windsor Castle.