I think I see where you would be torn both ways. Time/resources spent would be wasted, along with any emotions..otherwise I would think you would have left by now. Also I noted the bipolar/self esteem, and immediately thought of "white knighting/hero" behaviour and wondered if you were using that as fuel initially, and it has changed over time? Most relations change in some way, did you grow apart just by you both being comfortable and realize its not what you thought. I have had a personal relationship that lasted a bit over a year and it took that long for one of us to reveal the hidden side.
I want to thank you for making me look at this a different way. I realized after reviewing the last few weeks that actually she is in a process of complete meltdown, and I am doing nothing to help her. It completely exhausts me and your point on time/resources is well taken. I think the wake-up call for me is when some totally innocent remark by me made her run off then come back and throw her brand new cell phone against the wall...HARD. I mean hard enough to leave a permanent dent in the wall. It should have been clear then, but is absolutely clear now, that she is the person who comes into your home, starts grabbing plates, and throws them on the ground until her anger exhausts. She drinks like a sailor, smokes like a chimney, and simply cycles through waves of anger that get triggered by almost nothing. It's actually not an act, and as of today I am asking why am I part of this.
I think the trap for me was that initially all of these behaviors seemed to be due to lack of support from her friends. Now I understand she pushes everyone away, and she is stuck in a repeating and destructive cycle. I thought I could make a difference but I was wrong. If anything, I am feeding the pathology. I am incredibly loyal to people, so once I got caught up in this I did not engage in self-preservation.