I'm not quite sure. I relate to what you say about sometimes being more social. I am much more sociable, outgoing, talkative when my mood is 'high' and then when I'm low I will be quieter and more withdrawn, and may end up regretting some of the things that I did. I'm unsure if that is an 'unstable sense of self' though, as none of the fundamental things about me actually change, it's just that it depends on my mood how outgoing I am, but I still care about the same things/people, know what I believe in, what I like and don't like and so on. It's just that everything happens on a bigger scale when my mood is up! I have never considered myself to have an unstable sense of self; I know who I am.

Regardless of what you call it, it sounds like it's very distressing for you. Do you have a therapist or someone you can discuss it with?