The thing that makes me angry here is that I hang to an idea she will change in such a manner that life will be bearable again.
She will forever have her BPD npd
Both can be true --> change in such a manner that life will be bearable again + forever have her BPD npd
They can also be mutually exclusive.
One thing I will say in that if you consider going back, please learn the tools for how to provide a more stable life style for the relationship and family. This won't be wasted - if the relationship doesn't work out, you can use the tools in another relationship. I do that now, myself.
Last time that you went back, it sounds like that was enabling. No a criticism, just a statement that you don't want to do that again, that way.
Nothing changes without changes - which means both on you need to do work IF you want to try another round, otherwise it will be the same thing all over again. People often do this via a therapeutic separation, where a therapist sets rules to rehabilitate the relations and establishes how you interface and rebuild in the contacts during the separation (how you plan events, rules, etc.).
We have members that have rehabilitated a relationship. We have members that couldn't and the both partners went on to have other successful relationships. We have members where one or both had the same problems in the next relationship.
There are many possible outcomes.
Why do you think she has BPD. What stands out. What is it that attracts you to her? Is she diagnosed? Have you done marriage counseling? Is she in or has been in therapy?