Here's a thought. If you had no sense of self.....how would you even know if you were taking on the mannerisms of others? it's pretty common to adapt and try to fit in. Humans are social creatures after all.

Your "self" is in there somewhere, you just need dig it out somehow. Maybe you could try making a list and title it "who am I?" Start with basics, your appearance, then what do you like, what do you dislike? What do you believe, what do you not believe. What are you opinions about the world around you? What pisses you off, what makes you happy, what confuses you? If you could change the world what would you do first? It can get as esoteric or silly as you want. This is just personal exploration.

Doing this will begin to give you a sketch if who you are.

Hmmm, I kind of like this idea. Thank you for your post. I'm gonna do this myself.