Hey jillybeans. I have to say untangling who you are and who you are not is one challenging part of therapy.
Considering BPD persons sometimes lack a solid identity, always being who we need to be at the moment. It's something I have done all my life and poof people think I'm someone I'm not.
Anyway through the maze you do find your way again. I still cringe when I realize people don't/won't like me for who I am, really who I am. Then i stop and say well they're the ones who want me to be me though. Dang hypocrites! and I be me. Minding my behaviours that are truly negative and trying to make good habits come forth etc. Like with my new manager, I could easily just blow up at him for not knowing the right thing to do but instead I politely kid with him how easy it is to realize what the right thing to do is. I'm still being me and telling him off, but doing it in a way that is not confrontational.
Evilly I have had fun figuring out who I am. It's almost like getting to build yourself after you've seen the different models available.

and I can still be who someone else needs me to be in the moment using an as if attitude, like DB mentioned, but now I don't lose me when the moment is over.
BG said it pretty well, time it takes time. You'll get there, just have to give it time. Patience is good for stuff like this but if you don't have it then stubbornness is good too. Too darn stubborn to give up.