Her ex boyfriend (or whatever) is actually verbally abusive towards her according to her and her mother brings her down as well. She'll tell me stories about how her mom would point out all the things that were wrong with her as she was growing up and she would burst into tears sometimes while telling me this.
ok, you gotta be diligent here... if this is true about her mom and ex verbally abusing her then she may need you as a friend or, as Feathers put is so colorfully, an F-buddy. and it may be the case that you want to continue providing her what she wants to help herself deal with the pain in her life.
however, i've known some very shady women, and they're very good at creating drama and using it to get what they ultimately want (usually drugs, alcohol or "no strings attached sex".) i've had women use me to break up with their bf's who're abusing the snot out of them, and then left me out to dry immediately after with quick cut-off lines like "oh i don't think it would have worked out for us." be careful about about believing the things she tells you, she might be more confused and caught up in the lies she's creating than you are.
As for the insults; I'm guessing you have some social phobia being on here and all, and they will slowly chip away at all of your confidence. I was in a relationship with someone for a few years and for the last two or so I suddenly realised that I had absolutely no confidence left. Then I realised that it was because the person I hung around with most had stripped all of it away through constant day to day condescension and insults. I was there for when her friend's couldn't be bothered with her, and then when her social situation got good enough she broke up with me. A month or so after the breakup however her friends ditched her and without me as her standby friend and reciever of insults she fell apart. I imagine you're being used for the same purpose I was. We broke up a year or so ago and I feel better than I have in years. As others have said it's a really toxic situation to be in. You know that it's her behaviour that is unreasonable, but with four years of this it's going to get to you.
i got to this point also in my "toxic" relationship years ago... i had no confidence, and i'd sit up all night a nervous wreck. and four years, wow, you must have stone-hard skin man.