Why does this upset me so much PLEASE PLEASE PLEASE HELP!?

The reason it bothers you so much is because you feel you like messed up badly. I wont say that you did or didn't mess up badly, however you're just a kid so don't worry about it and move on. As for what to do when your friends talk about it, i wouldn't try to play it off and pretend like you don't care, i'd tell them that it bothers you because unless, they're a**holes, they wont keep bringing it up if it bothers you that much. If people you don't know bring it up just ignore them they're just probably looking for something to talk about because their living boring lives.