Why would people have such a far off base impression of you? It seems like it's most likely because you perpetuate that false impression by putting on a happy face and not showing your true feelings enough, if at all... otherwise why would they see you as a happy folk, of you're not one?
If that's the cause of it, I suggest changing your behavour to more accurately represent yourself. Then, you won't have the unhappiness of people thinking you're someone you're not, and you'll be closer to having people understand you.