And remind your partner they are going to have to earn your trust all over again. He DID do it again, and it hurt even more than the first time. Trust is the foundation of a relationship, and if there is none, then it's not worth the trouble, I don't think. It's not fair to be in that kind of relationship where you there is no stability. But definatly, don't let someone walk all over you.
Emotional abuse is the worst.
If you are in a relationship with someone who cheats you have a one sided relationship. Look at it from the other person's side, they are missing something from their life and they are going to get it. People who can have sex with just anyone don't need relationships. Relationships don't matter to them, just self fulfillment. I say, as a person who has been married or 10 years, find someone who is willing to commit to the relationship like you are.