I would give the following unexpert opinion.

1. It sounds like a very difficult situation. However, negative people are here to stay, so we might as well learn how to deal with them rather than trying to avoid them.

2. You can’t take responsibility for your friend’s decision to change. Whether she changes or not will ultimately be up to her, regardless of what you do. Your job will be to figure out how to take care of yourself within the context of that relationship.
That may involve setting some boundaries in the relationship.

3. You might investigate your own motivations. What is keeping you in this relationship? That might provide some insight into your best course of action. Are you simply there to provide support, or do you need her to need you in order to boost your own sense of worthiness and purpose? If the relationship is based on mutual neediness, be careful.

Finding a balance is in life is an ongoing challenge, one I work with every day. I have found that the difficult people in my life provide opportunities for me to increase my own understanding of myself and of people in general, and I like the ways I have grown through my difficult relationships. As the same time, I don’t go looking for trouble. Enough difficulty will find you in life without deliberately seeking more.

Hope this helps! Hang in there, friend!