Why do musicians do this to themselves?

I have been working on my grade 9 repertoire and technique skills for two years and today is the day that someone decides based on 30 minutes of playing if I get the piece of paper that says I pass the grade. I can see my heartbeat through my shirt and my hands are so sweaty I can't hold my phone. I'm imagining my fingers sliding all over the keys and playing all the wrong notes. I'm imagining completely forgetting the key signatures of all the scales I'm asked when I'm put on the spot. Waiting is the worst part, because all I can do while I sit here is imagine what about to go wrong. 2 years of work comes down to how I do today.