Zoso, I have been lurking on this forum for a good while, but your post inspired me to finally join. I, too, was involved (more like hijacked) by someone with BPD. For me, it was my best friend, although it may as well have been a marriage as we lived and worked together, spending almost every waking hour together as she couldn't handle being alone. Detaching was very hard, she was an expert manipulator and our friendship could be as wonderful as it was horrible. Like so many others on this forum, it was a steady build up of toxic behavior until one day, I just knew I was too exhausted and drained to continue trying to satisfy the demands someone who would never be sated. Ann put it perfectly as a "bottomless pit of her needs."
Sorry for so much backstory, but I just want you to know that you're not alone. This ended a little over a year ago, and I still am not fully recovered. Therapy definitely helped. It was amazing to talk to someone who could acknowledge that her behavior and the hoops I had to jump through were insane. I try and be grateful for the lessons she taught me; while she encouraged my codependent behaviors, I can now recognize them for what they are. I recently was seeing an ex who has struggled with addiction, and the second I realized he was still an active addict, I was able to detach gracefully instead of panicking and holding on tighter, wondering what was so wrong with me. The fact is, there is nothing wrong with me. I am a kind, generous, compassionate person, as I am sure you are too. However, now I can recognize when these qualities cross the line into being toxic for myself.
I won't lie, there are days that it is still very, very hard. I get angry because I am broke with no savings because I loaned her so much money that will never be repaid, and she is living off her trust fund. I gave up all my friends, because no one could understand why I couldn't be around her. Her family was my second family, I gave that up too. I don't regret any of these sacrifices. I comfort myself with the fact that I can sleep at night with a clear conscience, knowing that I have treated people fairly and with compassion, while she will continue to use and abuse people, forever burning bridges only to move on to the next victim. Someday, I will be in a happy, healthy relationship because I can take an honest look at myself and see what needs to change, whereas she will be miserable even in a relationship. I look at BPD as being similar to addiction in the sense that I don't blame her for having the disorder, but I do blame her for not doing everything she can to get help and learn healthier ways to relate. You don't drown by falling in the water, you drown by staying there.
Sorry, I didn't mean for this to turn into a novel, I guess I just needed to vent too. I hope you find something helpful, and feel free to PM me if you ever need someone to commiserate with. It's a long road to feeling completely whole again, but man is it worth it to be free from the toxic behavior and the crazymaking.