Hi jtree123,
Welcome to bpdfamily -- I'm glad you found the site. There are specific tools (communication, validation, boundaries, timeout) that everyone in a relationship with a person suffering from borderline personality disorder needs to master. People with this disorder tend to perceive the world differently than you and I, but there is an order and the rationale within that perception.
Have you tried validation techniques with her? Here is one explanation that helped me understand what it is (and isn't):
Validation and BPDThere is a lot more about validation on the site. I think it might help you in your conversations if you understand how validation can help her regulate her emotional responses.
Plus, this is a good perspective piece about what to expect in a BPD relationship:
The Do's and Don'ts for a BP relationshipI think it is actually fairly common with many BPD sufferers to lose themselves in another person -- you mentioned that your GF does that with you, and that it seems to be different than other BPD sufferers. Some of the suffering they experience is precisely because they mirror to such an extent, and this may feel like empathy. Without a solid sense of self, though, it is not quite the same.
I hope you keep posting and let us know how you're doing. These are not easy relationships, and people here know what you're dealing with. You're not alone.
LnL