Beloved Hollywood leading lady Diane Keaton died with a broken heart after never realizing her dream of walking down the aisle with the love of her life – her Godfather costar Al Pacino.

RadarOnline.com can reveal the failure of their longtime romance soured her on men for the rest of her life and that she was consumed with regret about what might have been with 85-year-old Pacino before her October 11 death at 79.

The pair was on again, off again for over 15 years before Keaton gave Pacino a marriage ultimatum in 1990 – and he headed for the hills.

“It’s sad that Al lived just down the road from Diane in Southern California for so many years, but there was barely a word exchanged between them,” said a pal.

“Al knew he had hurt Diane and figured anything he said would make it worse.”

Keaton admitted that she crushed hard on Pacino in 1971 when they played star-crossed lovers turned Mafia couple in the iconic mobster movie, but it wasn’t until they shot the 1974 sequel The Godfather Part II that they hooked up for a rocky ride that led to multiple breakups, and other relationships – including Keaton’s love affair with frequent collaborator Woody Allen and a five-year romance with Hollywood lothario Warren Beatty.

“Diane got back with Al in 1985 and insisted that this time it was for keeps,” said the friend.

“But when Al told her he didn’t ever plan to get married, Diane ended it. She didn’t want to be stuck in neutral with him for another 15 years.”

Even while Keaton nursed the wounds from the split for the rest of her life, the self-effacing Annie Hall Oscar winner continued to speak lovingly of Pacino.

“Al was charming, hilarious, and a nonstop talker,” Diane reminisced in an interview. “And oh, he was gorgeous!

“I was mad for him, and I worked hard on that one, but I guess I went about it in a not-so-perfect way.”

Neither one ever married, though Pacino went on to father four children by three women, and Keaton adopted two children in her 50s – 29-year-old daughter Dexter and 25-year-old son Duke, who are expected to receive the lion’s share of her massive $100 million estate.

But right up until the end, she openly wondered how different her life would have turned out if Pacino had agreed to marry her.

“She used to say, ‘Would it have been a better life? I would have liked to find out,'” a source revealed.