As I understand, and I am certainly no authority, the problem you are describing, and others share, is a reflex. It is triggered by sensory nerves under your eyes. My instructor called it the "mammalian reflex", which I do not think is correct, after doing a google search, but I bought into his theory anyway. His solution was to trian your body to react before preforming the skill test and as a good practice, train yourself before each dive till you can control the reflex. To train yourself, simply hold your face under the water, without submurging your entire head. First doing this while holding your breath. Once you feel comfortable with that, then do the same with the regualtor in your mouth. Breath in this fashion for 1 minute. Now your body will be less likely to "freeze-up" when to water hits your face. I have found this to be a good excersize before most dives, especially colder ones, to assure I have no problems under diving conditions.
Good luck.
Chris