not sure if I want this to be for the remainder of my life! Any response gratefully received ... thanks!
... and a baby is on the way.
Losing a child is a huge stressor and all of are at our worse when under huge stresses. pwBPD traits are at their worst for sure.
You sound like you could certainly understand and even accept this. You also sound like a guy who would be prepared for this when the next big life stressor comes down.
Possibly the problem you struggle with most is that this first life crisis has changed the tempo of the relationship and
you two have normalized some crappy dynamics into the relationship. She vents her internal chaos on you and you soak it all in and crash to the ground under all that weight.
With a baby on the way, you want to try to make this work. At the same time, you don't want to be an emotional tampon the rest of your life. I really hate that term, but it makes the point.
I'd focus on trying to slowly and methodically reverse the unhealthy dynamics. You must do the work to build the relationships in times of peace - when hell breaks loose - there is little you can do but wait it out and not make it worse.
People here can help you with that.