How can I stop being so sensitive?

I let the little jokes and such get to me. I know they aren't meant to hurt me, but they do. And there is this guy in one of my cles who is the most aggravating, annoying, person I know. I never give any justice because I refuse to stoop to that level, and I hate hurting people. I kind of go by the rule, "Treat others the way you want to be treated," but it doesn't really work. I'm still treated harshly. These little things get to me, and stay there, building up. I'm going to explode sooner or later, though I try to put it off. How can I stop letting these little things get to me? Before I do anything or say anything to someone, I have to think everything through. The consequences are never going to be worth the 'justice'. I don't want to change the image people have of me- I'm seen and not heard, nice, and quiet. I find that partially true. I'm getting tired of all of this, and at points I just want to break down and cry. I don't though. Help? I'm only 14.