Hmmm.... Being a good boyfriend is tough. Believe me, I know. If I have to put it bluntly, you've got to be chilled and laid back. If you feel like an argument is starting, stop yourself before you do, even if you value your opinion. If you ask her to 'come out' to you about what's troubling her and she says no, a simple "Ok" or "It's fine" or even "Sure. I'll be here when you want to, so just talk when you want to," is all that you need.
Being in a relationship is about sacrifice. Maybe, if you just sacrifice some of yourself to be the boyfriend she needs, it'll help out with your issue. Best of luck for both you and your girlfriend man!