Complex sentence: The song is a moderate up-tempo in which the narrator hears from a friend that his ex-lover had gotten married, and he calls and tells her that he finally realized it "could've been (him)" that she married.
Split into simpler sentences: The song is a moderate up-tempo in which the narrator hears from a friend that his ex-lover had gotten married. He calls and tells her that he finally realized it "could've been (him)" that she married.