Allie, I'm sorry you are feeling so badly. You do need to distance yourself more emotionally. Try to look at the situation as if you were a stranger viewing it. Your mom has disappointed you time after time, and that is not likely to change. There is not one thing you can do to change her. I think accepting that will go a long way towards helping you distance yourself from her emotionally. You have to do this for your own physical and mental health. Believe me, I feel for you. I've been there. But there's a point where your involvement becomes masochistic, allowing yourself to be hurt over and over. You deserve better. You may also need to get in touch with your anger about the whole thing and really let it out in a safe way, such as using a plastic bat to beat up a pillow, or going to a secluded place and yelling as loud as you can. If anger isn't expressed (in a healthy way), it will turn into resentment and depression.
Allie, I am proud of you. I absolutely think you are doing what is best for you and your family. I know it isn't easy.