When there are more arguments than happy times in your relationship, it's not worth staying in it. Like what others have said, if it's draining you down, then get out. Relationship for convenience will never work. If you can't commit, then don't. It will only get things worse. She'd expect more from you which you know you can't give to her.

I think what you need to do now is completely cut off ties with her. Don't feel guilty about it. You're only being honest to her and to yourself. Like what Simon said, turn off your phone. It will do you good. I think what makes her continue texting/calling you is she sees there is a great chance of you still continuing the relationship with her. If that's not what you want now, cut off ties. If worse comes to worst, change your number.