How to be comfortable just being ME?

How can I learn to just accept myself the way I am? I'm always putting pressure on myself to be a certain way around certain people. I act differently depending on the person I'm talking to. Sometimes this is a good thing, other times I get mad with myself because I know I'm not being who I really am. Sometimes people don't like me as much when I'm myself because I'm very quiet and withdrawn and I like to do my own thing, I think they think I'm snobbish or something, and maybe I am. But trying to be someone I'm not leaves me feeling drained! How can I learn to just be myself without feeling like I have to please everybody?