I don't really have any advice because I'm dealing with the same thing after about 8 months. Every now and then I can get him to admit that he flips out over little things and becomes very sarcastic/defensive and he says he doesn't like being like that and wants to change but sooner or later it always goes back to the way it was. I have learned that, until they want to change for themselves, there really isn't much you can do. If they change for you they will (almost always) resent you and if they don't, you end up resenting them. And if it's not for themselves, it will always go back to how it always was.

If this seems like something you can't get around then you might want to think about where this relationship is going. I have decided to allow my bf to choose his own rate of dealing with his issues. I have learned how to alter my communication or attitude when I can sense he is in a certain mood and I have learned how to deal with those times. That, in and of itself, has solved a lot. So even if he never totally breaks down those walls he has, I at least know how to handle it and can love him for who he is.

Hope this helps.