No, it's not all in your head. When I am stressed my mouth is as dry as sand. I normally carry around a bottle of water. If you can't do that there are mouth sprays that can help. The brand out here is Biotene. Visit your local pharmacy. They should be able to suggest something.

Can you mention your problem to some-one at the place where you do karate? Maybe they can suggest a solution. If not..... time to seek another place to bust your chops.