So I am speaking from my personal experience here as I also had a friendship like this but not as bad. My friend also had BPD after some very traumatic events. So some of the things I say may seem hash and hard to do but they are possible.
I'm not going to tell you to cut her off as I think that you will find that too difficult to do (and quite frankly if you could stomach that you would have done so already). Though if you think you can maybe taking a clean break from this friendship is what's needed.
However, it is important for you to remember that you're not responsible for her happiness or well being. Repeat this to yourself self like a mantra. Yes she is unwell, but you cannot fix her so to act like she is your responsibility is detrimental, for example by taking phone calls through out the night when you need to sleep as pregnancy is exhausting.
I agree with the poster above that you need to pull back but set times when you will speak with her. This is important for two reasons, first you don't feel like you are completely abandoning her, second it regains some control of the dynamics of the relationship. It is important that you try take back some agency, it will make you feel better. For me resetting the terms of the relationship was far less stressful than being the sounding board for everything that my friend got into her head. Often they were sad things, negative things, but there is a reason that therapists get paid money to deal with that day in and day out.
It's ok to hold some part of yourself back as well. If you don't have time to talk be honest and stick to that. Hang up the phone if needed. Practice saying firm but friendly good byes in the mirror.
Friendships are give and take and would you be happy treating someone else how this friend treats you? Would you be happy if someone else treated you soon to be child like this?
It will be hard to set boundaries, she may not understand or try to guilt trip like she seems to have done previously. That's ok, repeat the mantra, continue to reinforce boundaries.
When I did these things I found that it was like a weight lifted from me. It did take a couple of months but the practice did shine through. The other thing was I found I was more willing to be assertive in the rest of my life as well so that was a nice bonus.
I think you sound like a lovely person, good luck