You are staying with this clod for the same reason many JW's stay with the religion long after it is obvious that it isn't the truth. It's a case of "Better the Devil they know....than the Devil they don't know" They have no outside friends left, they are convinced they are sinful and flawed and it makes them feel extra virtuous to be somewhat of a martyr for the cause.
Instead of wondering what's wrong with your boy friend or if you are stupid, you need to ask yourself:
1. Do I in someway believe I am at fault or somehow responsible for his actions?
2. Has he isolated me from my friends and family to the point that without him I have no one?
3. Has he or someone else in the past belittled or criticized me to the point that I truly believe I can't do any better?
4. Why don't I understand that this man is constantly finding ways to sabotage himself or our relationship and that I deserve better?
This is an age old problem and you aren't the only one to find herself in this situation. I'm glad you are reaching out to others to help you gather the courage to do what you know you have to do. Once you are free of this situation and have found someone who deserves you, you'll will look back and wonder why you didn't act sooner but mostly you'll be glad you did.