Keaton — who passed away on Saturday in California at the age of 79 — played Al Pacino’s girlfriend in the “Godfather” trilogy, and they started dating in 1974. They broke up in 1990 after Keaton gave him an ultimatum and he refused to commit to marriage.

A source told the Daily Mail on Sunday that Al Pacino, 85, now wishes he had married the late actress back then. A friend of the legendary actor said, “I know he will regret forever that he didn’t take the step when he had the chance.”

He added, “For years after his split from Diane, Al used to say: ‘If it’s meant to be, it’s never too late to try again.’ But unfortunately, it’s too late now.”

The friend continued, “Looking back, Al admits that Diane was the love of his life, whom he always described as an ‘amazing woman.'”

The source also mentioned that Keaton and Pacino never spoke, even though they lived near each other in Beverly Hills, adding, “I once asked him why, and he told me: There’s no need to talk to each other. We said everything we needed to say at the time.”

Keaton recalled her relationship with Pacino in a 2017 interview with The Sunday Times, saying that in hindsight, it was better they never married: “I didn’t even want him to propose. I just thought maybe he would marry me eventually.”

She added, “I thought, ‘Don’t propose — let’s just do it.’ But that never happened, and that’s a blessing for both of us. It would have been a nightmare for him.”

The Oscar-winning actress told the newspaper that they ultimately were not right for each other. She shared, “We’re both very eccentric, he needed a woman to take care of him, and I needed a man to take care of me… It was very important to leave each other alone and say goodbye. But that wasn’t my choice.”

Keaton previously admitted she fell in love with Pacino after admiring him during the filming of “The Godfather” in 1972.

She told People magazine about her former lover in 2017: “I was crazy about him. Charming, funny, a nonstop talker… He had a lost orphan side, like this crazy naive genius. And what a handsome man!”

Keaton never married but adopted two children in her fifties — daughter Dexter (29) and son Duke (25). Pacino also never married and has four children: Julie Marie (35), twins Anton and Olivia (24), and son Roman (2).

Pacino made a rare public appearance at the American Film Institute (AFI) Lifetime Achievement Award honoring Keaton in 2017. During his speech, he said to her, “You’re an artist, Dee. You’re a great artist… I love you, forever.”

Pacino was not the only former lover of Keaton reportedly struggling to cope with her passing. Reports said esteemed filmmaker Woody Allen — who dated Keaton for a few years in the early 1970s and worked with her on several films — is “very upset” over her death.