That it wasn't all about how others perceived me, but how I perceived myself, especially in these two areas:
1. I spent a lot of time in high school worrying about my body and physical appearance. I wasn't even overweight--I had a nice figure. But I obsessed over the size of my chest in comparison to others, the shape of my thighs, the size of my nose, and so on. To me, what's sad is that one of my most vivid memories of high school is a guy telling me I looked a little chunky. I mean, come on! Why did I give people like that the time of day and give credence to their words? Who cares if my chest was smaller. Who cares about the size of my thighs. What about my sharp wit, my strong laugh, my big heart? THOSE are the things I should be concerned with, size-wise. THOSE are the things that matter most.
2. I also spent time worrying about my popularity. Dude, when you leave high school, no one in college or out in the real world cares who you hung out with. It's utterly pointless, LOL. Hang with the band nerds if you want. Become president of the chess club. Be the top cheerleader or a football star. The point is, do what YOU want, what works for you. Don't do stuff because you're worried about fitting in. Those 4 years of high school go mighty fast...and they're awfully short in comparison to the rest of your life. Don't let that time period define how you perceive yourself.
I realize it's easy for me to dole advice in this manner. Honestly, all I can share is my own perspective on things. But you know what? That's ok. Part of the struggle, the drive of being a teenager is learning SELF-definition. So take my advice with a grain of salt and forage your OWN path. To me, that would be the best option of all.