Hi SE,
I understand so much of what you shared. When an adult is enmeshed in unhealthy ways, it can feel as though the BPD behaviors must be addressed through both parent and child. Not easy.
With my SO's D19, I found it was essential to tap into compassion in almost aggressive ways, if that makes sense. I read books that walked an important line between building compassion and building relationship skills. I also practiced mindfulness like my life depended on it
and actively carved out time to get myself centered. What was so challenging is that D19 brought the skill levels down, and SO met her there, instead of raising the skill levels.
When one partner is intent on boundaries and the other actively undoes them, it can feel like you're holding the hose to fight the fire, and the other person keeps turning off the water
My T actually brought in a child psychologist to our couples counseling. The purpose was to impress upon SO the real effects of not having boundaries with D19, to help him see how his behavior was harming not helping.
It made a small but important difference in the dynamic.
What has had a bigger impact is me recognizing that SO is very codependent when confronted with strong emotions he does not want to feel. When he is in rescue mode with D19, I try to gently point out what are essentially avoidant behaviors so he can see the pattern. His mom died and he kept asking me if I was ok -- strong emotions are hard for him to experience and I try to turn his behaviors into expressions of what are often difficult emotions. Anything less than gentle, and he digs in his heels so I often make a simply point (usually, a validating question) and then move on.
Like you, I also make choices to take care of myself when he can't. In counseling, it was agreed that he would consult me about any big decisions involving D19, and when he doesn't, I remove myself from the situation. Same for rescue behaviors when it's supposed to be our time. Getting him to agree there was a problem was a half step, bringing to his attention when those problems were happening another half step. The next step was the two of us agreeing to handle boundary issues a certain way. When he regresses, I have a response that requires no argument or discussion, I simply do it because it's based on prior agreement when he was feeling strong.
And last, recognize him when he does the right thing. He is seeking the rescue route because it gives him a sense of self worth that he probably didn't get as a child. WHen you recognize him for doing his best, you interrupt that script and divert things toward positive behavior where he may very well need to feel praised and appreciated.