I think you have General Anxiety Disorder (GAD) like I did. I was always anxious too (still am but much less).

http://psychcentral.com/disorders/sx24.htm

Maybe you could do some reading for a bit if you're nearly finished school, then see a therapist and discuss it with them. I don't think you'll be dismissed as only having school worries and stress at your age, you have enough experience with school that you should be okay with school.