It wasn't that you were not good enough, it was that you were not exclusive enough. You were too laid back and he did not feel the grip of your relationship. Exclusive relationships are supposed to be possesive. You are supposed to be reasonably jealous and possesive of your love. You are supposed to let him and others know that. Then, if he does not respect that, it is all on him. If you feel badly because your pride was hurt - don't. If you feel badly because you love him and are afraid of losing him if he does this again, then make that clear to him. Make it clear to him that you cannot go through this shit again. He is at fault, it is up to him to commit and forge a new relationship with you. Also, it is up to both of you to punish this other girl by some legally acceptable means. She is an interloper and other women should be concerned about her lack of respect for other people's relationships. 

I'm not trying to excuse what he did.  I understand that it hurts you. This is not because there is anything wrong with you. Truthfully, most men will fuck most any woman. It's just the way men are. Most men learn how to suppress that kind of behavior in a relationship. But most me do think about sex  with most women. The fact that your guy hasn't learned to keep his dick in his pants is his shortcoming, not yours.

I'll give you a suggestion about how you can start the feel better.  To do that I need to find out a couple of things. First, do you and your boyfriend have a good sense of humor?  Secondly, what's the wildest thing you have ever done sexually?  I am not asking for details, just a one sentence description. Also, do you view yourself as having low self esteem?