This is a very hurtful thing to discover. But, you know it's over now. What do you want to hear from him when you confront him? That's an important question to ask yourself. It's easy and natural to want to hurt him for the hurt he's caused you, but couldn't that put your whole relationship at risk? If things are going well maybe now is not the time to confront him. Maybe he was struggling when you were a few months ago and he made a bad decision that he has since regretted.I suggest that you proceed with caution and think more about the future and less about the past.