First off, let me say that I can seriously relate to your situation. My best advice to you is to try to pinpoint what exactly is setting you off. Make a journal or something if need be. That may shed some light on which stressor is causing you 'too much' stress. For example, if you come right home from work and your boyfriend says or does anything remotely provoking, maybe he is not the cause of your frustrations, but instead something that just happened at work. Often, people do not realize that they are lashing out at the wrong time and place, as well as, toward the wrong individual(s). After awhile you may see a pattern, or you may not, but it is a great start and will surely reveal something of interest. This would be a big step towards solving the problems it's causing you since realistically you just can't lighten your load so easily.
It may also help to try to view your boyfriend more positively, since I assume he supports you. Try to remind yourself daily of his efforts and you may, in time, be less prone to lash out at him. Communicating with him about your stress would be a great way to encourage further support from him as well, which in turn will counteract your daily stresses. He may go from a negative to a positive outlet.
Sometimes it can be as easy as looking into the future when you feel like you're about to snap. Keep in mind that tomorrow may be better and less stressful and that today was simply today...if that makes sense.
I hope I helped in some way. I really feel for you, as I have a very good idea of where you're coming from. Good luck.
edit: A journal itself may also relieve built up tension.