Who am I? I mean like, really?

It seems like I am a different person every day. One moment I'm the greatest, and then in a minute I'm the worst. I don't really change that much, but I keep acting like a different person to different people. I feel like I just keep trying to please others instead of myself. I want to be popular, but I don't think that's the key to happiness. Sometimes I feel like a helpless little girl (I'm not really that little, I'm almost 12 years old and like 5'5) and some days I feel like a tough girl ( I have a black belt in tae kwon doe.) How can I stop feeling like this?