I can’t responsibly answer your question without talking with your friend directly. I can only make some general guesses.
There are several reasons why a person might act this way. She may be terribly insecure and need an exclusive friend to hold onto to feel okay. In that case, therapy could help her develop a stronger self-esteem and more inner security. She may lack the skills for managing multiple friendships — in which case she needs some help to learn more about social relationships. Or she may have Borderline Personality Disorder, in which case she would benefit from Dialectical Behavior Therapy. Or it might be something else.
If you are interested in helping your friend and she is open to it, you could suggest that she see a therapist for an evaluation and some help. Running through one friendship after another must be painful for her.
It might be helpful to you to think about whether you could have done anything differently to avoid getting pulled into such a one-sided relationship so you don’t make the same mistake again. If this is a pattern for you, some therapy might be helpful for you as well.
I wish you well,