Al Pacino and Diane Keaton were broken up for years before she passed away … but, he says all the memories have come flooding back since her passing.

The legendary actor released a statement about his late costar and ex-girlfriend — with whom he shared an on-again, off-again relationship from 1974 through 1990 … admitting news of her passing shook him to his core.

Pacino calls Keaton his “partner” and friend … and says she changed the course of his life on more than one occasion.

Al says although more than 30 years have passed since their split, the “memories remain vivid, and with her passing, they have returned with a force that is both painful and moving.”

Keaton “lived life without limits,” Pacino adds … and, he reflected on her legacy … pointing out she’s left a mark on the world that will not soon be forgotten.

Diane had several high-profile relationships … she also dated Warren Beatty … and Woody Allen, who released his own statement about her passing earlier this week. She never married … she once said her relationship with Pacino ended when she gave him an ultimatum demanding he pop the question.

As we told you … Diane passed away Saturday — and, we now know she passed away from pneumonia. She struggled with another mystery illness for years before her death, and a longtime friend told us they weren’t surprised when she passed away.

Dispatch audio from the morning of reveals there was a “person down” at the address — confirmed by sources to be Keaton — and, she was rushed to a local hospital, though they weren’t able to save her. Keaton has already been cremated.