No apologies needed for writing long posts

(I hope not, or I'm in lots of trouble! )
This is a place to share, to get things out, and unless there is a software imposed limit on the space, I don't think anyone else has ever said much of anything about it, so please, write way.
I guess a big question to ask right now is:
are you together at this time/still engaged? because, not to sound callous, but if you are not together, and she is possibly pregnant by someone else, I don't see this as really anything you need to overly concern yourself with (other than the fact you obviously still care for her). I just think that elephant in the room needs to be pointed out and addressed before anyone can chime in to see how best to offer any advice.
What medication was she on, before the pregnancy? BPD is an emotional disorder/behavioral one, not usually treated with medication, but other issues, like depression and bipolar disorder existing WITH BPD can be.
Is she in the care of a therapist/OBGYN/MD at this time? Have you been able to speak to them?  :)o you have a therapist of your own to help you make your way through this situation?
Are you living in the same town, now? Or still apart? Is she still "kicked out"?
She is basically doing the push/pull. She got mad at you for "abandoning her" when SHE moved away (No, I know you did not abandon her, but she likely had an unrealistic expectation you'd move with her that she never communicated). So, she sought out emotional supply from other men both to fulfill her needs for attention and to punish you for not being there. Cheating is not uncommon with BPD. Meeting new people they can show only what they want to is intoxicating. Once the new people see the "real" them, they are often abandoned and need to move on to a brand new person, or seek out a comfortable old one. Shame from cheating will make her treat you differently, as she blames you for her feeling shame. Toss in an apparent history of body dysphoria that resulted in an eating disorder or obsessive exercise, and you have a lot going on. And to top it off, she is not dealing with pregnancy hormones.
You've got a lot on your plate at the moment.
What would you like to see as a goal with this relationship?