Tired of relationship...

My boyfriend and I have been dating for 2 years. We're both 20 and going to college. Recently we've been arguing more and more things about him irritate me. We always argue because he always wants to have sex but I've been going through a lot of diet and lifestyle changes due to health reasons and almost never feel like having sex. There are other things, such as his pretty bad depression he refuses to talk to anyone about that also sour the whole deal. I'm not sure what I'm doing anymore. We had been planning on getting engaged this summer and married the next after graduation, but now I'm not sure what to do. I guess I just needed somewhere to get this all out/beg for advice. (additionally he's going into the air force right after graduation and I wouldn't really be able to use my degree I've worked really hard for)