I used to not be able to breathe through my nose, only my mouth. I ended up having my tonsils and adenoids removed so I could breath better. Now I breathe through my nose just fine. :)
If it's nothing like that, and you are just used to breathing through your mouth...then just try closing your mouth, and *sniffing* air in...like a deep breath through your nose. Try to stop yourself from breathing through your mouth. Use only your nose. Eventually, you will be breathing through your nose without even having to think about it.
Well, you may want to talk to your doctor about this. If you are trying to breathe through your nose, and you feel you are not getting enough air, then it could be your adenoids. They are two things in your nose, and sometimes people have over-sized adenoids (you can't see them), when they get them removed, then they feel they can breathe better.
You need to seea doctor for this problem, its possible that your nasal passages are not wide enough or that you had an injury as a child that was not addressed properly and is causing you trouble now. Only a doctor can help you with this though.
That's my problem though. If I breathe through my nose for long periods of time, I just feel like I'm not getting enough air. I'm sure it's all in my head. My nasal passages aren't blocked or anything. I'll try this though, thanks. =]