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Forgiveness is often tied to trust. You would have to trust that he is truly regretful by realizing that he had made a mistake and then that behavior has to halt. He would also have to own the responsibility for his actions and explain what may have led to this sort of behavior. It is possible to forgive and it is usually a process that you go through over a period of time. In order to forgive, you'd also have to deal with your own feelings that you've experienced as a result of his betrayal, alter your internal self dialog (if for some reason you were blaming yourself or felt inferior, etc)
People are prone to making mistakes including him. In the end, holding onto resentment is going to hurt you more than it hurts him. In a way, you allow yourself to forgive in order to move on whether with or without him.