@fearmenot, it's been a pleasure to be here for you, though all I've really been able to do is read your posts and give hug reacts. But I've been thinking of you.
In fact, I got a book about caretaking for narcissists and BPD, and I was so hoping there'd be some good stuff and I could recommend it to you, but it wasn't good enough that I would recommend it as a resource. Regardless, I understand so well trying to understand someone else's stuff, be empathetic and supportive, and then have a wall thrown up and run right into it. I spent so much emotional and other energy for years on folks with those issues, and I get not only how draining and crazymaking it is, but how one can get sucked in again by someone new or even by the same folks when one is compassionate and have the inner resources to help others...only to have the offer denied and then taken in a manner that's not offered, or get little rewards here and there to keep on giving, followed by abuse.
I did pick up a helpful term from one resource that's make a huge difference for me: slot-machining. Returning again and again to the slot machine of hope. I now catch myself when I'm in slot machine mode, softening up and ready to go love and serve that person some more (and maybe they'll finally get better!), because now I remember the teased jackpot never hits, I don't get the love back, I don't get the tacitly promised rewards.
Sending empathy, compassion and respect.