I will say this: protecting yourself (physically, emotionally) is never selfish. Especially not in a situation like you describe. Even if someone is sick, that does not make you obligated to offer yourself up as a whipping boy -- either figuratively or literally.
What you describe is not normal or acceptable. And while medication can help, if she doesn't get real, dedicated treatment (like Dialectical Behavioral Therapy) with someone experienced with BPD, it's unlikely to improve long-term.
Its natural for you to be devastated about her condition and her treatment of you. And I'm sorry you've experienced all you have.
And I know what it's like to feel like you don't know what "normal" is anymore. I was in that spot last year with my husband. He was emotionally and verbally abusive and I got to the point I was believing all the horrible things he said to me until I finally had a moment of clarity.
We have an article on domestic violence that might have some useful information for you:
https://bpdfamily.com/content/borderline-personality-disorder-and-physical-abuseAlso, do you know much about FOG (Fear, Obligation, Guilt)? We have a workshop about it here:
https://bpdfamily.com/message_board/index.php?topic=82926.0I hope you'll take a look and let us know what you think.