I'm frustrated because even though I practiced all day, David still beat me.
Maybe you should think about hiring a better teacher to work with?
I'm not sure where to find a better violin teacher around here.
Accepting defeat is part of the competition, and learning from your mistakes will help you grow as a person. Try practicing more and putting an effort into improving rather than trying to compare yourself to someone else.
It seems that he cheated and was professionally coached, it's not fair.
Keep practicing and working hard, you'll get better, it just takes time. Believe in yourself and you'll be great one day.