It's better to forget because if he left you, he's probably only going to go back to you, not because you changed or anything. If he did, that's rare, that's like super rare. But he's probably not going to go back to you because you changed. And change will take a few years or two. But you shouldn't go back to him if you broke up with him because he's not going to change. Because if he cheated on you, he's going to do the same thing.