When is enough just enough?

Well me and Ashleigh had a huge blow out tonight, it's only been a week since our last one.  I'm SO tired of it, I don't want to give up on us but I also wonder am I staying in a relationship that is never gonna work and where things are never gonna change.  I just don't know anymore, I feel like we've been having the same argument for two damn years and nothing is really any different.  She acts like a total bitch and yells and cusses at me for nothing, expects me to just sit there and take it and not say a damn thing, I get pissed and stand up for myself and tell her I'm tired of her shit, then she cries and tells me how sorry she is and that things are gonna change.  But here we are again, back at the same place we always find ourselves.  I really do love her, I really do want to make things work but I just don't know how.  Maybe we just aren't meant to be and I'm hanging on to a relationship where I'm never going to be happy.  That's all I want, to be happy.  I don't know why I can't seem to make relationships work, I'm sure some people would say part of my problem is I keep going back to the same people over and over and over, then I'm surprised when we end up having the same problems.  ARRGH I just wanna scream, I'm so frustrated.  I just want to know when is enough just enough?