Hi St6123 and

You've landed somewhere with people who get it about how stressful BPD relationships can be. When there are kids involved, too, it's a whole new level. My H has two kids, and their mom has many BPD traits and behaviors, so getting support from this site has meant a lot to me.
First things first -- are you
sure she's pregnant, and if so, are you
sure you're the dad? Answers to those questions can have a huge impact on what's the most effective way forward.
I think I'm reading that of her 3 kids, one is in high school -- how old are the other kids? Is there any conflict between your GF and her kids' dad, or is that pretty stable (functional coparenting)? Do you interact with the kids' dad at all?
...
Staying in, and having a livable, relationship with a pwBPD often isn't intuitive, and can take some new tools, skills, and approaches that a "generally normal" relationship might not need. It sounds like you've already experienced that explaining things to her (which can work in "normal" relationships) doesn't seem to help yours, and sometimes makes things worse.
If you haven't checked out our article on
What Does it Take to Be in a Relationship with a pwBPD, maybe start there, and see what resonates with you, or what sounds do-able that you haven't tried yet.
This is hard stuff -- we'll be here for you.