Most Helpful Girl
Has he given you any further info on the girl? Who she is, how he knows her, etc? Could it actually really just be a friend? Of course it isn't right that he wasn't open to you about it, and it isn't surprising that you cannot trust him anymore.
I don't know if you should break up. I think you need to think about it for a bit but if you cannot regain your trust there isn't much point in the relationship.0THIS IS NOT RELEVANT ANYMORE