Not sure if this has been brought up before but have you ever been seen or treated depression? Sometimes this holding on to negative things is a classic symptom of depression. When we are depressed, we only see the suffering, the pain, the heartache around us and those we love.
We don't allow ourselves to feel happiness if we are depressed because frankly, depressed people use the depression as a comfy blanket. She has almost become that blanket for you (which I've thought is the reason you cannot get beyond this.
Its that chicken or egg thing: are you depressed as a result of the relationship or depressed and the relationship (with all its chaos and ongoing drama) is what you gravitate towards.
As someone who suffers from depression I can tell you that when I am not following my treatment and taking care of myself, I tend to be attracted to negative things. When I do feel good, I want more light in my life, not darkness, pain or chaos.