Well, FYI, the "I never intended to hurt you" thing actually makes things worse. I've experienced this from the other side of it, and in my opinion, that just sounds like "It's not my fault you got hurt." That doesn't fly.
Apologizing for it is just step one. What you have to do after that is try to mend the enormous hole in his trust and his confidence. It's a terrible blow to a man to have his girlfriend fall for someone else. It does grievous harm to his self esteem. Apologizing does nothing to address that, nothing to give him any assurance that you won't go off and do this again next year (like BrianK's girlfriend does every so often).
Can you think of some things you SHOULD have said to him before it was too late?
