Yes... .soul searching is the answer to figure out what you want.
Education about the traits of BPD and the r/s dynamics will then allow you to make a wise choice about is what you want possible.
I see those as two totally different trains of thought.
So... .if you love her and want a r/s... .you need to educate yourself along those lines. You have to be wise enough to know that even your best efforts may not be enough to change her choices.
Even in your current situation... .my gut says there is a dynamic that could change... .if you change.
Now... more to your questions of why:
For me... .for better or worse means something. If I didn't lay it all on the line to uphold a vow I made... .I would be violating a big personal value for me.
I've also got kids with her... .there is a family to consider.
During the good times... .she is fantastic. Never met anyone close to her... .there is hope.
And most recently... .and good for me and the future... .I stay and make changes because she is making changes. Momentum is going in the right direction.
I'll finish off by going back to soul searching. Be deliberate about this... .don't rush it. Making a clear decision about what you want will be empowering.