I'm the same way. Anxiety makes me draw blanks and stutter. No amount of mental effort or preparedness seems to make it go away. Funny how age changes you; I used to be pretty good at exams and tests in school.
Personally I'd hate to go through the university process and make it all the way to PhD only to choke on the exam and be denied the qualification. That's a lot of time, money, and effort lost on such an archaic practice.
I know this may not sound very helpful (I too resisted at first), but have you tried to seek specialized help? A good therapist should be able to help you deal with whatever issues you have that could be causing the condition. It's like hiring an experienced bug fixer.
Personally I'd hate to go through the university process and make it all the way to PhD only to choke on the exam and be denied the qualification. That's a lot of time, money, and effort lost on such an archaic practice.