Well.... You practiced hard and all you can give is your very best (depending on the circumstances)..... if you're comfortable with that then you can still go for it..... who knows.... perhaps you are better than you think you are.
Just practice, study and play as much as time allows you to. You know how this works, I don't have to tell you. Come tournament day, just relax, concentrate and play your best every set. Don't let anything work your nerve.