I think that it is perfectly understandable to go back to unhealthy relationships. I, myself, have done this on multiple occasions. I'm not sure if it's the same for you, but I have this internal need to trust the people I'm around even if I know that I shouldn't. And, I also try to remember all the good things that I've had in these relationships.
But, while this is all fine and dandy, I found myself surrounded by people I didn't actually really like. I found that I didn't really have anyone to talk to. So, I did still stay with those people, but they do not play large roles in my life. I don't bring them close to me, because I know that they're just going to hurt me. But I'm not really one for completely deleting people out of my life.
In your situation, I think you should either talk to your 'friend' about the situation, which could turn against you, but also improve your relationship. Or, my recommendation, distance yourself from her. Find someone new to talk to. And if you can't find someone you can trust in person, online friends can help you just the same.
I believe that you are well aware that this person isn't going to be someone you want to keep with you forever. But, by cutting her off, I think you may cause some unnecessary turmoil, and she could play this against you. Again, this is your own decision, but really I think it's just safe to distance yourself from her and find better people to be close with.
