Well, not wanting to sound insensitive here, but you literally drove her away! Trust is a HUGE component in a relationship, and you seem to have NEVER trusted her. I am a firm believer in still having your 'own' friends even when in a realtionship, because if something happens in that relationship and you don't have any friends, then you're all alone. And, you were depriving her of this! I don't really blame her for leaving, I'm actually shocked she put up with it for as long as she did.
You need to do some soul-searching, and get over your insecurities before any realtionship is going to work out for you. If you still love this girl like you say you do, you HAVE to start trusting her! Let her hang out with her friends (single or not, male or female) and dont' give her any grief about it. Show her that you really do trust her and love her. But relationships always come crashing down if one of the partners is controlling, sort of like you were. If you do indeed want to work things out you really do have to change your controlling ways.
Never regret something from your past, everything happens for a reason.