My girlfriend is stressed and overwhelmed and takes it out on me.

Let me start by saying I've been with my girlfriend for about a year. I love my girlfriend, I care so much for her and want to be there for her always. She works full time, goes to school full time and is a single mother. Her stress sometimes kills our relationship and makes me feel like crap and sad. There are many times that I am afraid to ask her a question because she just comes back with attitude like I'm annoying her. I know she is better than this, I've seen her happiness and its so wonderful, but she is just so overwhelmed right now, her happiness is hidden. I really want to help but she just pushes me away and it turns to a fight and to me feeling sad. Sometimes I just want to say I'm done being your emotional tampon and move on, but its so hard when you care so much for someone. But at the same time I wonder why I put up with it. I think my answer is I don't want to be alone because when things are good they are great and wonderful. I just don't know what to do.