Do something even if it's swim or play basketball. As a teen you have this slice of your life to devote yourself to development, not just academically but also in your sense of confidence, balance, and discipline. You may or may not make any real friends, but whatever you do, do it for yourself. I am a shrimp but am not afraid as an adult. That was the +. The - is that I have major aches and pains from old injuries. I was from the era of starvation and screwed myself over growth-wise and emotionally (well, my family was dysfunctional, too). I'm into Jiu Jitsu now, at age 50, and I'm thankful the young me wrestled. Just don't let others control your life, tell you to starve, shame you for losing, or say you can't do something. Also, it is okay to lose. I used to get depressed over it.