thats one of the toughest things about BPD, and loving someone with it.
its worse under stress, and stress is poorly coped with. and that, in turn, makes things especially difficult for everyone.
loving someone with BPD is a bit of a balance between being their rock, their consistent support, and also setting reasonable limits, and not trying to stand them up, but allowing them to stand themselves up. this is a good start when it comes to better understanding our role as loved ones:
https://www.bpdfamily.com/content/what-does-it-take-be-relationshipwhat do you think? is that something you could try?