I feel gross in front of people?

(im 16 female) for some reason when i go to school for example my day always goes wrong. i do have a prob with worrying about what people think about me and im working on it and i usually leave my house with confidence and pride in myself thinking im gonna be myself and not care what anyone says.. but once i get to school i feel not just ugly but also annoying and stupid, and like a little kid basically. i sweat a lot or i have bad breath on my good days which makes me act akward and makes me feel even worse about myself;(