I can relate to you in a similar way in that whenever I go to the GP for blood pressure or something similar, purely because of the situation I think my BP rises a little. I dont know a way out of this other than to try and remain calm and take deep breaths and relax a little etc - but as you know this is difficult.
Why not try giving up coffee, alcohol, (cigarettes if you smoke) and salt for about a month before the test and see if that helps - also try some more excersise.
Best of luck
Maz