Hi flamingspiral
I saw a lot of posts before writing this that helped me not feel alone, so I hope I can also help with my own venting.
Certainly. And you aren't alone. When I was in the later part of the relationship with the pwBPDSO, boy did I feel alone. Before I discovered this community and all the resources, I had heaps of confusion and aloneness. Yes, there were people I knew that went through tough stuff, but nothing like on this community. So yes, participating here--it helped me not to feel alone too.

Yet I am haunted by what ifs and this delusional notion that things will ever work out with my ex in spite of everything I rationally know. ... .I still feel and struggle with.
I think this might help.

"What if" it could have worked. Absolutely yes! I felt heaps of that too. So there is a what if. The thing is, with a pwBPD, that's just it; you've had, have, and will have this person with BPD. OK, so when you accept that it's really a want,
your want, and you were denied it, and that creates a bigger want, then let it flow through you and pass through you. It will pass through you--yes it will. A bit like kicking a bad habit, that's what the relationship was like for me here. There might be some weaning involved, but your heart will "get" it.
If you only rationally know something that involves involvement of the heart, then the heart doesn't seem allied with the cause of the head. So rather than "fighting" it as you suggested (basically fighting your heart), another way is that you be self-compassionate by recognising that your heart really wants this specific relationship, and give it time to understand that this relationship cannot be had with the cooperation of the pwBPD. Give your heart the space to maybe be anxious, hopeless, whatever comes for the moment. It's not actually that bad, it actually can sometimes feel great to be "free". Whatever it is, it will pass.
None of us should be ashamed for still being hung up on them months or years later. These cluster b's are as intoxicating as they are toxic.
Yes, of course it felt that way for us, but your heart will catch up if you keep practicing. There might be some weaning involved, but your heart will "get" it.

I hope you find peace.