hi c12!
How long have you been dating?
I understand a bit of what you're going through, though roles are flipped. My MIL has BPD and for a host of reasons, I don't interact with her anymore. My H felt trapped between his mother and me. The most helpful thing for my H and I to read and understand was the Karpman triangle. Check out this article and let me know what role you think you, your mom and gf are playing:
https://bpdfamily.com/content/karpman-drama-triangleFrom your perspective, you might say you are the victim, your mom is rescuer, and gf is persecutor? Gf might say that she is victim, mom is persecutor, and you are rescuer? At different times, can you see yourself playing both roles?
Based on the role you think you're playing, what can you do to help yourself get off the triangle?