I am truly sorry for your situation. I have never been in your EXACT situation - dealing with so many huge challenges with a child. So I am not sure what advice to offer.
That said, I have tried to help another adult of lower consciousness before - only to my ruin. It almost broke up my wonderful marriage. In the end, I chose to kick the other adult out of my life. I still feel guilt because I believed that I was the only one who could help him - or willing to - and he had no one else. But I later realized, that it was HE who needed to help him. So letting go of the guilt was my first step. I believe that I have healed from this very painful part of my life.
Based on how you worded your pose, it looks like you've been trying to find balance - caring for her and yourself. It doesn't look like you have the support system in place for a "balanced solution." From your words, I sense that you feel you are at this alone.
If you are at this alone, I see you have two choices:
1. you either throw yourself into the fire and try to pull her out at the risk of losing your own sanity. You may succeed, you may fail. This is a risk, but you must be willing to pay this price if you choose this path.
2. walk away and recognize that you are making a conscious choice to NOT RISK this price - the price of your sanity. And FORGIVE yourself.
Either way, you must recognize that you, and you ALONE, have the power to create your own happiness - to end this situation.
If you chose to walk away, it will be painful. People may call you names, they may say bad things about you. You cannot live your life for other people at the detriment of your own! If you stay and change the situation, you may lose yourself - which sounds like it's already happening.
It seems to me you have some very big decisions to make. You need to just make it and accept what follows - as it doesn't seem like there are any other roads for you.
I'm truly sorry. I hope that what I've said does not cause you great pain.
I know how it feels to feel responsible for someone and then realize that I was not willing to give my soul for them. And I did have guilt and sadness for a long time - and he turned out even worse without me there to be his crutch. But I have learned to forgive myself and move on.