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.
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!