I've been with my gf for almost a year. She was diagnosed years ago with borderline personality disorder. She told me this early on but I never realized how bad it really is. What do I do when the person that she "idealizes" isn't me? It's a guy she been obsessed with for almost to decades and he doesn't even want anything to do with her. He only paid her attention when it was convenient for him to get laid and that was only sporadically over the years
Did your gf feel that way about you in the beginning?
Regardless of what this other guy did, this might be a pattern for her. Whenever things in her current relationship issues pop up, she goes back to idealising the other guy.
How does she treat you? What sorts of issues are popping up in your relationship? Have you read up a lot on BPD?