When you're in over your head and start questioning whether you're actually capable of what you're trying to do, just remember:

Success isn't getting things right the first time (though it's wonderful when it happens)

Success is persevering until you get it right. Stop rushing life, and stop pushing yourself too hard, just take things slow, try and learn.

Exams are daunting, but they're a counterproductive way meant to get you to learn. So breathe, actually do your best, and if you don't get the result you're hoping for, try again. Part of life is facing and overcoming challenges. Good luck.