OP, my daughter has BPD - and whilst she doesn't (yet) have a child, I do know that there is help out there. When was the last time you spoke to your care co-ordinator/counsellor? Your Psychologist? They are your first port of call and there will be help available to you, even if just The CRISIS Line (which is "open" 24/7, from my experience).
No one is going to take your baby away from you.
But they will help you.
I left an abusive relationship before my daughter was born, and was diagnosed (wrongly) with PND, on top of bipolar, 21 years ago now. If I managed to "keep" my child, then you will, too.
What about family support? Friends? I can't quite credit that there's no one you can talk to/trust in this situation.
Asking for help, is the right thing to do - not only for/by yourself, but also for/by your daughter. You both deserve support and, OP, as "unMumsNetty" as this is, I just wish that I could give you a hug and tell you that the only way is up, that you deserve happiness, that your daughter is going to be okay, just so long as you keep loving her and asking for help.
Just because your GP apparently shut one door to you, doesn't mean that they all are.
PM me if you need to.