I Care Way Too Much About What People Think About Me

Does anyone have any advice on dealing with this irrational fear I have? I worry about how others see me ba<x>sed on how I act and what I wear. I don't act like I would with someone I know well, and I don't wear what I think I would like to wear. When I'm with someone I know well, I can be outgoing or fun, but with others I'm quieter than a mouse. I see other guys wearing way better clothes than me ( I dress so plainly) and I sort of get envious. While I may want to wear what someone else is wearing, I get a great fear that someone will be thinking "oh my god he looks terrible in that".
What do you think about all this? :/