The Montreal Gazette published letters written by famous personalities where they wrote letters to their 16 years old self. I read a few and I thought long and hard about what to write, without being too harsh or judgemental (I really hated my 16 years old self...).
Dear 16 years old self,
I wish I wrote this letter to you when you were 13. There is so much I want to tell you. Those first few years of high school were quite rough, but things do get better though. Here is a piece of wisdom, be confident! You're a really strong kid, you just don't realize it. Believe in yourself and don't let those jerks in class get to you. Don't doubt yourself and do what you think is right. Although you are very loud (12 years down the line, you will still be loud, don't try to change it, it won't happen) and strongly opinionated (that will increase once you build up the confidence level) you will have a lot of friends who will appreciate who you are and value your opinions and friendship. And for the love of god, don't give up on your geeky nature. It's who you are, don't fight it, embrace it!
28 years old self.
