How to reduce stress and stop blanking out?

Hey during the December math section I was extremely stressed- possibly due to having 3 math section in a row including an experimental section (I think), but I just didn’t want to flop like last time. So, I am here pleading to you how to reduce my stress levels and feel more confident while I am taking the math section. I also need suggesting on getting that clear mind feeling so that nothing but me and the test are in the zone.
But also, I want to tell you that I started not being able to comprehend some of the questions, looking at them not close enough and ended answer the wrong thing complete (only happened during the grid ins when I was getting really stressed), as in I would read y=2/3 as y=2/3x and just plain out doing things wrong like exponent rules that I 100% knew because I excel in school.

So to summarize: Sometimes I would just blank at the question even though I understood it comepletely, and at some times I just couldn’t keep myself composed.

I empathize with you. I was also extremely stressed during the last SAT test, probably because on the math sections, I kept blanking out.
First of all, if you practice enough, whether it is math or reading or writing, the questions will be a breeze. If you already know the concepts, then the stress derives not from getting the concept, but from being worried about not finishing in time or not finding the answer.
When you take your test, keep an open mind and don’t get stressed on one question. If you don’t get it, or you try out that problem for at least 1 min, move on. Don’t get hung over that problem. I suggest finishing all the easier problems before spending the last 5 mins on that problem.
Don’t forget to breath. I know this sounds silly, but when stressed out people take tests, they hold their breath for a really long time (only letting a little air in and out but again this circulation is minimal). Take deep breaths and think happy thoughts. Motivate yourself that you will get the solution/answer to the question!!
I don’t know if this applies to you, but when I was working on the same problem you had, I started to freak out about not finding the answer, and began chicken scrawling in the booklet. Try to write neatly, and show ALL the work. This is one of the main reasons why I will miss a step in the problem, or misinterpret a variable for a number etc. Like your algebra teacher taught you, write everything neatly out. This will benefit you in the long run when you get the correct answer on the first try, rather than attempting to solve it with 3 unsuccessful tries.
I hope you consider these not so new tips when you take your next SAT test.
Also, if you intend to practice, try taking the test in a completely different environment. Go borrow a study room in your library or school if they have one, or sit someplace new so you still feel the pressure from the new location.

Best of luck!