If you're reached tertiary level you are quite intelligent
already.
Exams aren't to stress you, it's to test
what you know. Just take it as that. (A lot of people told me this
actually, but it didn't help me, however I hope it helps you.)

Listen to Baroque/classical music when studying, if possible before
your exam day, or just before the exam. Works for some people. I
wasn't a great performer at school but everytime I studying I had
classical music in the background to help me relax. Then when I sat
for the exam, I could sort of imagine that music playing in the
background, it helped me relax. I recommend Pachelbel's Canon in
D.
You could also suck on a sweet just before the exam, it may calm
your nerves.
You can also reinforce yourself with positive thoughts during
studying and during the exam. Don't condemn yourself. Just keep
saying things like, "This is easy," "I can do this..." "I know
this, I just have to think it through a while and it will come to
me..."
Regarding passages, I think all you need is focus. Skim throught
the intro and conclusion to get the main point of the passage,
before you go on to the body. Write down your thoughts as you read.
Similarly, when writing something, think of your conclusion first,
write it out. What do you want to tell the examiner? The main
points? Then you can work upwards from there.