I think you are just anxious, try not to focus on the negative.
The lack of confidence is the surest way of creating anxiety. I
used to suffer from the same problem especially with exams, I one
day realised the problem exists only because I am not well
prepared.
So if you want to pass an exam aim to score 70% that way u have a
20% buffer for failure. If you want 90% study to achieve 100%. But
take it one step at a time.
A good way to simulate this is in competitive sports, very often in
competitve sports you find yourself under pressure, if you have
practiced your moves often enough you will find that your reaction
under pressure both motor and mental is instinctive, the
unconscious mind takes over.
The same thing is true in life and in exams, if you are good enough
with what you do, everything seems natural. The problem is when you
are not quite there.
So my friend, the root to anxiety is a lack of self-believe.
Unfortunately my friend the only way to convince yourself you are
good is to work your f_cking nuts off.
Sorry, no easy solution, go to GP no use, he can't make you smarter
and better at what you do, hard work and diligence will though.