Advice...... BREATHE. If you tense up, so will your horse, and then it will all go downhill. When you're looking at your patterns, memorizing them, add in places where you will breathe. I mean literally, you go as you're learning the pattern like this -- "Okay, this is jump one, this is jump two, go left. Breathe. Here's jump three, go right. Breathe."
That way, if you forget to breathe and relax, you will have those marked intervals where you remember that.
Also... Don't think of it as a huge show. Just think of it as just another local horse show, it's all about having fun. The more relaxed you are, the less mistakes you'll make. Sounds like you have the potential and ability to do this! If you made it this far, don't freak about it.
Good luck! Tell me how it ends up!
To rein a horse is not only to guide him,
but also to control his every movement.