I'd like to say that a lot of what it being said in this thread is pretty hurtful. I have Borderline Personality Disorder. I was diagnosed when I was 15. I have been treated for it with standard behavioral treatments (i.e. talk therapy, not chemical therapy). I regulate my behavior and, for the most part, am successful at checking the destructive and manipulative impulses I have. On the occasions when I fail to check them, I accept responsibility for my words and actions and do my best to apologize sincerely and repair any damage I have caused.
I am not incurable. I am not a monster. I am not toxic. I am not a drama queen. I am an ethical, polyamorous man with Borderline Personality Disorder.
People with BPD have some often over-looked skills which, when decoupled from destructive behavior, make us excellent partners. I am very attuned to the needs and emotional states of my partners. I am attentive. I find deep gratification in the happiness of others.
Ok, end of soap box speech.
Having wrestled with the disease all my life, I do have some suggestions about this specific situation:
1) Give your SO "Stop Walking on Eggshells
". It's the best book out there to help people without BPD learn to recognize the patterns and what the motivations are.
2) Make your emotional boundaries clear. You have to put your own emotional health first, and your SO should be receptive to your requests for support.
3) Anytime your BPD metamour starts trying to use you as a pawn, politely ask that you not be brought into the issue. Say something like "I understand the issues, but I need to take care of myself before I involve myself in this." It's important to make sure the subject of your sentence is always yourself; that way the metamour can't accuse you of telling her what to do, etc.
-First of Three