I am in a similar situation with my bestie. She is bipolar/borderline and together we are a two headed monster. On her own, to me and others she is abusive and unapologetic. She is intolerable to everyone but me. For my part, I like the time we spend together and the understanding and acceptance of each other's "crazy". I can take a bit of inconsiderate, abusive, selfish behavior to also have someone who appreciates me for EXACTLY who I am.My therapist and my boyfriend have warned me about the dangers of our friendship and how clearly triggering it is. She and I are TROUBLE together and the effects are lasting.
I have cut her off before but I always call her back. I'm deeply lonely without her.
I'm sorry if this isn't helpful. I just want to say that I understand and that I'm glad you posted this, because I could clearly use the same help!
Hang in there.