Why can't I leave him?

I don't get what you mean by a really nice guy? Really nice guys don't act like that. They do go out with their friends and drink during the week sometimes (particularly in your age group, well, in your age group pretty much all guys go out with their buddies and drink during the week), but they don't jump into heated arguments and say things they'll regret (often).
You can't make him change and you don't have to. What's in the past is in the past. Even if you screwed up at the start, he's made a choice to stay with you. Everyone makes mistakes. Move past that and look at your relationship. I agree with PickUpMan in that you're problem may well be one of confidence and self-esteem. I like that you have said that he doesn't deserve you. Good for you. Go with that and let your sense of self worth grow. Past that, follow PickUpMan's advice as it is right on the money.

Sounds like you need a bit more confidence. If he is right for you and wants to be with you, lay out the ground rules. Tell him what is acceptable and what is off limits. Don't give him much leeway. If he screws up big once, let him know that you are gone next time. When he does it again, get gone.

If this relationship continues this direction, it will royally suck going forward. Rarely does anyone change who they are and if they have to do that, they will resent you for making them.

Why do you have to forget the good times that you had...recall them fondly and move on to more and better times with the right guy. Might be a bit tough at first but in the long run, you will be very happy that you took all of these steps.

Sincerely,
Dr. Joyce Bros.