Who said "I know it was you Fredo, you broke my heart. You broke my heart"?
The person who said, "I know it was you, Fredo. You broke my heart" was Michael Corleone. This line is spoken by Corleone, played by Al Pacino, in the film "The Godfather Part II" which was directed by Francis Ford Coppola.
John Cazale as Fredo Corleone—of the mafia Corleones—is Michael Corleone's older brother. Although Fredo is the elder bro, he's always been the Luigi to Michael's Mario—second fiddle, relegated to the sidelines, seen as weaker. But Fredo wants to be player #1, so he betrays Michael by making a business deal behind his back.
When Michael finds out, he gives Fredo a big kiss at a party in Havana. This is the kiss of death, and he follows it by saying, "I know it was you, Fredo. You broke my heart." Betrayal leaves a bad taste in Michael's mouth. As the somewhat new Don of the Corleone crime family, Michael is protecting the family business in the aftermath of an attempt on his life.