Cara, this is for FUN! Remember that. How cool will it be to run along next to your horse and POP! she's up by your head, and then POP! she's up there again and you're halfway around and here comes that oxer and POP!
What a blast!
Set goals for yourself when you go in that ring. Instead of thinking of all the things you have to do or that could go wrong, concentrate on two things: Remember the order of the jumps, and get your horse to go over them!
Worry about style the next time, about not looking silly or falling on your face or getting the hiccups...the next time. This time, just make it around the course and get out alive.
I suck at prep work, to be blunt, but discovered the first time I really did it correctly that memorizing the course frees you tremendously to worry about other things like actually getting over the obstacles instead of trying to remember where they are. Don't just memorize the diagram- memorize the diagram, then memorize which jump is which by glancing up from the diagram, then run it in your head. Seriously! Close your eyes and picture what you'll see in front of you as you go around the course. Don't pay attention to the bird's eye view, be there in your sneakers trotting around with your horse. Where do you look after that jump? Do you go right or left? Know those things, and remember where the tricky places are with tight turns or spooky stuff. When you can do that in your head without thinking about it, you've got it down.
Good luck! Oh, and if you can, watch the other competitors do their rounds first.