I have a nervous facial twitch?!

I'm 15. I've had it for a few years now. It makes me twitch me upper lip. I usually get it when I'm stressed or nervous or just generally upset. I'm actually not sure if it's a twitch or not because I can control and make myself not do it but it's very difficult. I basically got rid of it over the summer holidays because I was never stressed but now my work load in school has gotten very heavy so it's come back with a vengeance. I feel incredibly self conscious about it because it's on my face. People don't generally mention it but I know that they notice. It isn't something that happens occasionally, it bothers me pretty much all the time and I hate it.
Is there anything I can do to stop it?