Welcome to the nuthouse!!

I think what you've described is one of the more difficult aspects of bpd to overcome, if it can be overcome. I say that, because who we are is developed from the time we're born. I wrote something that dealt with this not to long ago, take a look if you like, and don't let the weird title fool yah..

http://forums.psychcentral.com/showthread.php?t=244296

I do think dbt could help with taking on people's problems, and their personality.. but I'm lost on how you find your own stable / consistent personality.