Underneath, don't forget (in your moments of guilt) that she has created this situation that she's in, not you. No matter what life circumstances have driven her to become this way, she has always had a choice about what decisions to make, and she has chosen the most damaging path rather than a wise one which benefits herself and those around her. You are NOT responsible for that; there is absolutely NO reason why you should ever feel guilty for having mixed feelings towards her. Guilt achieves nothing and causes so much unnecessary harm.
It is highly unlikely that she will change; but one thing that you cannot predict is how, over time, you will change as a consequence of the decisions you make in regard to her.
I have personal experience of this, with a close relative who was unpleasant throughout my life in many ways but I chose to stand by and care for them when they were ill (and, ultimately, dying). After a few years I was suddenly hit in the face by the realisation that during the years I had helped this person I actually learned to love them for the first time in my life. When they eventually died my grief was not complicated by guilt.
Now, I'm not saying this to suggest that you should lay yourself down as a doormat for your mother - god forbid that you should ever do that (and that's certainly not what I did)! Each person and circumstance is unique, and you are the best person to determine what is the wisest course of action for your situation. But whatever you decide, it will change you, in ways that you simply can't predict.
Ohhhh, life is such a bloody bugger sometimes. But IME when we make tough decisions we grow SO much as a result - so remind yourself that with time, you'll be benefitting in some way.