Possible porn addiction

My boyfriend and I have been together for almost 4 years. We've had problems in the past like any normal relationship but there's one thing that always comes up. He has an issue with looking at other women or watch porn on his computer. I've expressed that it makes me feel crappy when he would rather look at other women instead of getting intimate with me.
When we've talked about it all he says is he has a problem or he's just a crappy person. It makes me feel crappy that he doesn't want to fix his possible addiction but also just puts himself down when I try to talk to him about it.
What can I do?

1