First, don't set a per-question timer. The real test doesn't time you per question, so it doesn't actually help to do that during practice. Give yourself a full block of time (based on however many questions you're doing).
Next, this anxiety will still probably manifest as you get towards the end of your block of time. So we need to deal with that. Your symptoms are significant enough that I suggest trying meditation:
https://www.manhattanprep.com/gmat/blog ... mat-score/But here are some other techniques as well:
http://www.manhattangmat.com/blog/2012/ ... anagement/The biggest suggestion I have is to remind yourself that (a) you do mostly know what you're doing, and (b) when you hit something that you can't do, the best response is to guess and move on. I don't mean that just from a practical standpoint. I also mean that from a testing standpoint: the test is designed to see whether you can recognize when you do NOT know what to do.
Back to the anxiety symptoms. It's possible that your symptoms are severe enough that you will need to speak with a psychologist to come up with coping mechanisms. Try the meditation first to see how that works.