I feel very nervous. How do I get over my nervousness?

I'm feeling really nervous now and I have a queasy stomach. I'm always feeling nervous or stressed out and because of that I always have stomach pains which affect my attendace at school. The reason why I think I'm nervous is that I think that my best friend is going to ditch me because she's been spending a lot of time with this other girl. I know that I have some security issues but what I really want to know is how to get over the nervous feeling because my stomach is killing me right now.