A short personal story if I may. I have sat for 2 formal piano exams in my life. Grade 10 at 45 and ARCT at 47. Musically it was the most exciting thing I've done at the piano. The day of my Grade 10 I drove to the exam centre and walked around the parking lot for 10 minutes contemplating running away to the south seas to hide the shame for fleeing.
I went in.
Two years later I walked in again. By this time I'd invested 5,000 hours of practice over 4 years, I was ready for it to be over. I didn't give a hoot about what was about to take place.
It went well.
My teacher Leon knew what he was doing: preparation is the first step. Knowing you are prepared reduces anxiety around fears of competency. Secondly, he gave me permission to make mistakes. That really helped. As a piano examiner I can confidently say nobody gets 100%. You won't either. So the question is "what will you do when you make your error?"
I say forgive yourself. Keep moving. You are a champion for being there and the examiner is on your side.
BTW, no adult piano student of mine has ever failed an exam. As an examiner, students do fail, but I never failed an adult.
Cheers and good luck.
ps Check this out. 89 Year old pianist passes his grade 3 piano exam.