Don't worry...
This happend alot on my OW course, resulting in quite a few people bolting for the surface if the mask failed to clear on first try or water went up nose.
All advice mentioned so far is excellent, personally I found 2 things really helped me:
1. RELAX, with the mask off take you're time to get used to the sensation and breath normally. Put the mask back on BUT before you try to clear RELAX and take 2, 3 or more steady breaths.
2. With your mask off breath in through your reg but breath out through your nose, I don't know if this works for any one else but it cured all problems I was having.....
Exhalling through the Reg does result in turbulence around the nose, which I personally found uncomforatble, some people hold their nose but this is impractical as you will need both hands to replace your mask and so will have to let go eventually.
Anyway hope this helps, everyone has problems at the start.
Dirk