Billy Joel Attempted Suicide Twice After Affair With Friend's Wife

Billy Joel is recalling a heartbreaking period in his life ... revealing he attempted suicide twice in his 20s over the guilt he carried after having an affair with his best friend's wife.

The music icon dropped the bombshell confession in his new documentary, "Billy Joel: And So It Goes," which premiered Wednesday at NYC's Tribeca Festival. Notably, he was not present.

In the no-bars-held revelation, reported by People, Billy said he admitted to having an affair with Elizabeth Weber -- the wife of his pal and former bandmate Jon Small -- when Jon became suspicious. At the time, they were all living together -- which is when BJ said he fell "in love" with her.

He recalled ... "I felt very, very guilty about it. They had a child. I felt like a homewrecker. I was just in love with a woman and I got punched in the nose which I deserved. Jon was very upset. I was very upset."

Billy said the overwhelming guilt led him into a downward spiral -- he lost his closest friend, he began heavily drinking and he had no place to live because he had been living with Jon and Elizabeth. At that point, he decided being dead was his best option.

His sister, Judy Molinari, was working as a medical assistant at the time and gave him sleeping pills to help him get through the nights -- but Billy says he decided to take all of them at once.

Judy reveals in the doc he was consequently in a coma "for days and days and days," adding ... "I went to go see him in the hospital, and he was laying there white as a sheet. I thought that I’d killed him."

He wound up in the hospital a second time after drinking a bottle of "lemon Pledge."

Jon is the one who brought him to the hospital the second time around, and Billy confirms his pal saved his life. The gesture also saved their friendship.

Jon notes ... "He never really said anything to me, the only practical answer I can give as to why Billy took it so hard was because he loved me that much and that it killed him to hurt me that much. Eventually I forgave him."

Billy notably reconnected with Elizabeth later in life -- and they were married from 1973 to 1982. He tied the knot 3 more times, including his marriage to current wife Alexis Roderick.

Billy's shocking confession comes just weeks after he cancelled his upcoming shows due to his diagnosis with a rare brain disorder.

He is currently undergoing physical therapy, and TMZ learned he plans to get back on stage in the future.