When you let someone upset you, you give them power over you, it shows that their opinion matters to you. Start by pretending you don't care what they say, save the angry for later when they can't see you. Maybe roll your eyes at them and huff, so they think it didn't work. If they know they bother you, they're more likely to do it again. Unfortunately, it does still hurt when people say mean things, but as you get older you get desensitized to it, it bothers you less and less and you get tougher.