1. Stop trying so hard getting people to like you. My early adolescent years were spent trying to be "part of the group". I wanted to be accepted and liked and to have people want to include me. Those same people (mostly girls) that I thought were so popular and important and wonderful are no longer part of my life. I don't use Facebook, so I have absolutely no idea what happened to them or the kinds of people they turned out to be as adults. I don't miss them and in fact I never think of them.
2. Be more comfortable in your own skin. Like many girls, there was a list of physical things that I did not like about myself. The bump on my nose, my big front teeth, my long feet, my this, my that. The piece by piece scrutiny did little for my self esteem and caused me to wish for differences over which I had little or no control. I would tell my young self that NOBODY cared. If I spent more time just having a good time rather than worrying about looks or proportions or comparing myself to others who were completely different body types than me, I would have had a much more enjoyable adolescence. We are a whole package, not a collection of parts.
3. Don't be embarrassed by your abilities and successes. I was raised to be humble, not boast, not draw attention to myself. I downplayed getting good grades, or being able to draw well, or excel at public speaking, or any number of things. I received little or no praise from my parents who had already raised three other children by the time I came along and were actually probably too old to have the energy to put into raising another. I would tell my young self that I was actually smart and good and talented and that it was o.k. to be proud of myself.
4. Have more fun. I was a serious child, somewhat shy and cautious. I've never wanted to look like a fool. I spent a fair amount of time around adults or just playing on my own. I would tell my young self to throw caution to the wind and just have a blast, learn to laugh at myself, take chances, don't care what anyone thinks and just have fun. Pure unadulterated joy -- that is what I would recommend, no matter how silly or foolish it might seem.
To sum it up, I would quote Bill Murray in "Meatballs", "It just doesn't matter! It just doesn't matter! It just doesn't matter!"
Do you have anything you wish you could tell your young self?