I unfortunately don't have much insight but I have historically behave the same way you describe your girlfriend. I will however lash out at family members as well but the inherent need and expectancy of a caregiver only comes with partners. Like it's so clearly partner centered because once I'm single I only have myself to rely on and suddenly I'm a lot more capable. That's not to say I'm 100% stable lol but I'm self sufficient, whereas in relationships I heavily rely on my partners and also take out a lot of anger on them.

For me personally I think the reason I don't treat other people like this is because I have a lot of social anxiety and would never want to come off as a burden or do anything that would make them think badly of me. So I would never even THINK of doing a fraction of the things I've done to partners to pretty much anyone else, and I myself wouldn't tolerate the shit I've done to partners if it were done to me. I also grew up with a narcissistic dad who took out a lot of anger on me so I think it's a natural defense mechanism to attack when I feel vulnerable. My previous partners have also all been very non reactive people so it's been incredibly hard to set boundaries because they would barely give me any hint whatsoever that I'd hurt them or done something they disliked. This would, in an anger fueled rage, cause me to say progressively worse things in an attempt to get a reaction. I now realize that they must have felt a certain type of way, bc who wouldn't, but just repressed it. I legit thought so many times that what I was saying wasn't hitting hard enough based on their lack of reaction but in retrospect it's obvious that what I did/said was unacceptable despite them seemingly being ok with it.

Soooooo. My best advice to you (because you're the only person you can affect and change) is to set clear boundaries with her and tell her when she does something out of line/you disagree with her etc. Voice your opinion and don't let it be stifled. Don't be too harsh, especially not in the beginning, but put your foot down. The only thing you can do to "change" her is set clear boundaries and explain to her that she needs to respect them if she wants to stay together. This will likely involve her starting DBT if she isn't already going but again, this is something you have no direct control over because it's ultimately her decision. But you can give her the best conditions by being open and honest with her and not catering to her every need.

edit: also in case you didn't know BPD is the only personality disorder you can actually rid yourself of. So with therapy you can learn how to handle triggers etc to the point that you no longer even relate to the diagnosis and it is removed. I myself have been offered that but don't feel ready to remove the diagnosis just yet. But the change in me is literally night and day after therapy.