keeping calm

Do some searches on this topic. There's been a lot of discussion about this subject, including this post: https://www.beatthegmat.com/test-anxiety-t207.html

Check your timing progress regularly and have a plan for what to do if you discover you're behind. In general, you should know how much time you should have left at question 10, 20, etc.

Let's say you discover at, say, question 20 in quant, that you are 6 minutes behind. That's 3 questions. You have 17 questions left in the section. You have to make that time up somehow, and you DON'T want to be forced to guess on the last 3 questions in a row (or simply run out of time).

Instead, spread your guesses out over the remaining questions. 3 out of 17 means a guess approx. every 5 questions. Specifically choose questions you think are very hard (of course!) and deliberately spread the guesses out. So if you guess on 21, don't guess again until you get to at least 24 or 25. And how do you know which ones to choose? Simple: when you're reading it for the first time, if you're thinking, "This is really hard - how would I even start?" Bingo - random guess, minimal time, move on.

Also, another trick I like to use. Sometimes, I second-guess an answer after clicking confirm (when it's too late!). Occasionally, I'll even realize that I might have mis-read the question or clicked the wrong answer... too late. If you find yourself dwelling on the past (defined as any question that isn't on the screen right now), tell yourself that you'll think about it... just as soon as you've finished the one that's on the screen right now.

Don't tell yourself not to think about it - you'll just think about it more. Give yourself permission to think about it - in just a minute, when the current problem's done. And keep doing that until you forget about it (or the test ends, whichever comes first).