Insecure boyfriends

My boyfriend got drunk last night and broke our bathroom mirror with his fist... He said some things that just completely stunned me. My boyfriend has always been an extremely sweet guy. Super out going. Very out there and loud. He talks non stop constantly. He's that cool guy that everyone wants to be friends with. That random girls will throw themselves out. He does get a big cocky sometimes and he can also be taken as egotistic when he's being too much. But I know who he really is. He went out last night and got completely shit face hammered. He was throwing up and mumbling stuff. Then in the bathroom I heard our mirror break. I went to see what was going on and he had punched it and it shattered. I asked wtf was going on. And he yelled "I HATE HIM!" I say "Who?!" And he said "This person I have to look at every day. I hate his fucking face?" It took me a bit to realize he was talking about himself but he was saying things like "Nobody likes him. He's just a worthless POS who everyone pretends to like!". Then he started crying saying how much he hates himself and how annoying he is and that he wished he was someone else and every time he looks at himself in the mirror he gets sick and he truly hates his own face.... I have never in our entire relationship seen him do anything like that. Today he carried on like nothing ever happened...

I don't want to leave him. He wasn't angry towards me. We've been together 4 years and he's never done anything like this.

My boyfriend doesn't normally drink. He only went out drinking because a friend was in town. It's not like he gets drunk every night and constantly breaking things. I don't want to throw away 4 years over a mental break down.