Feel like i can't breathe? i'm 18, otherwise healthy, and don't feel sick at all? am i just too focused on my breathing? what's going on?

Anxiety: Probably , if you had a physic exam and all is fine , you probably are reading too much of something which may be normal. Is thereany thing which makes you anxious or you are worried about something ?

Possibly: Anxiety and stress can interfere with breathing. Usually people with that type of problem will complain of not being able to take a deep enough breath. Have you seen a doctor to rule out any kind of lung problems? I would certainly do that first and then if it is anxiety you can learn relaxation techniques to help.