A tip the learning specialist at my school used to tell us- take the first five minutes of the exam to write down all of your anxieties (what you're worried about, or just in general what you're feeling). Write it in the corner, or in a margin, or on a piece of scrap paper. Sometimes it helps to just get out all of the tense energy.

If you had diagnosed anxiety, is it possible to talk to your teacher about testing accommodations? You may not end up using them, but it's definitely something that is worth investigating. I do not have test taking anxiety but I have extended time and a quiet testing space for other reasons. I rarely use the extra time but knowing that I have it reduces any anxiety that I have going in.