I think he has cheated and he's trying to explain it to her. He can't lie and it hurts him just as much as its hurting her because he truly loves and cares for her. It was something that got out of hand. Good intentions turned obscure. (angels cry, when stars collide)They were soul mates, but the stars collided the minute he made that choice to cheat. Now the angels cry because something that was so perfect has been ruined.