relaxation

I desperately wish I could change the fact that I'm so sensitive about everything. For example...I hurt my knee at our last track meet and ever since then, it's been hard for me to even jog around. Tonight at lunch I was sitting with my legs tucked under and someone made a comment "wouldn't that hurt your knee or whatever?" me taking everything to heart, responded with "yeah i'm actually faking it" sarcasm of course. Following that was a comment "you shouldn't take everything to heart so much slack, I learned about that and that means you're really insecure. Maybe I am...how do I change that? I need to lighten up, big time. Ugh