Boyfriend of someone recently diagnosed with borderline. I dont know what to do.

My girlfriend and I have been together for 9 years and are now on the brink of our third breakup.

We had a textbook borderline relationship according to books and publications about the topic. Things started out very rough but over the years it has gotten a lot better and now we adopted living in seperate apartments to give us space and prevent things from escalating too much. That has worked great over the last few months and breathed new life in our relationship.

But yesterday we had a very nasty fight over a miniscule matter and now she wants to throw everything away again. We worked so hard for that progress and headed the right way. Ironically yesterday just a couple of hours before the fight we had one of our many deep reflected talks and she said herself how happy she is to have a boyfriend like me and how much our relationship improved. Now she talks about me having to find someone else and that she has to accept living on her own for the rest of her life because she is not capable of having a relationship and I am struggling very hard to keep calm and collected. All these accusations and going back and forth took a toll me to be honest and I dont know what to do now.

I dont want to force her to stay with me, but I just cant let go because her reasoning just doesnt make sense is in such a contrast of her behaviour and our dynamics in the last months.