I have extremly low self-esteem how can I fix it?

I'm always self-conscious around certain people. And when i get self-conscoius the way I deal with it is by being quiet and pive. People describe it as me being there, but not really. Like if my mind was somewhere else. They also describe it as ditzy, but I know I'm not ditzy because I'm in high cles and I'm smart. When it happens I feel like I'm not really there and I can't focus. But this only happens when I'm self-conscious. How can I stop caring about what others think, be more like myself, and not come across as ditzy? Please help this has been bugging me for years. Thanks :)