This is hard. I would probably tell myself to be a little bit more confident and not to worry about what other people may think, as long as I am happy then my true friends would be happy too.
As a teenager you have a lot going on emotionally and physically. You think you know everything, the world is against you, you are insecure, hate something about your body and it is probably one of the hardest times of your life. You don't really listen to any advice that is given to you but if I saw a future self I hope I would sit up and listen.
Personally I didn't have traumatic teenage years. I was average at everything I was lucky enough not to be bullied, I wasn't troubled with horrendous acne and my friends were true friends who did stand by me through some tough times. I was one of the lucky ones which I am thankful for.
So I think I did hold myself back a little by being unconfident. I was nervous about doing things a little different with the fear of rejection and being 'laughed at'. Even now I like to do things to the best of my ability and I am reluctant to do things which I may not do well in and I get nervous about starting something new.
In saying that I think it is important to step outside your comfort zone every so often and do new things. It helps to keep you interesting and does help to build your confidence. I need to take a leaf out of my own book!