So many things to say about this, all of which a good coach should be telling you already.
Firstly, anything can happen on the day and nothing is a foregone conclusion. Who said you cannot win the whole thing? Ok, reality check.. your unlikely to at your first tournament, but stranger things have happened.
The first thing you need to get out of your head is the gap between your rankings that is clearly playing on your mind. Remember that player was also once where you are and they have managed to get close to the top 100, who says that won't be you in 18 months time?
You may find that there is a player that is new to the circuit and turns out to be a complete natural and unknown sensation. It could be this player that causes you much trouble because you are so worried about the other guy and you do not pay the respect this sensation deserves.
And so on and so on....
At the end of the day, focus on what you can control by doing what you did to get you to the nationals in the first place.
The best thing you can do is use this experience to learn like David wrote.
Dan made a good point in another thread the other day... Take a note pad, write down after the match what happened, such as...
What you need to work on
What you did well
What did the player do against you that worked really well for them
Was there anything that they got easy points from you on?
And most importantly, what one thing can you learn from you opponent.
Oh, and do it straight after the match, DON'T wait otherwise you might forget that all important thing
