Just breathe. The best thing you can do is breathe. Find something else to do, study your course, so on. Just keep yourself occupied, and try breathing exercises by counting to 3 before inhaling/exhaling again – I know that when I get nervous, it makes me a bundle of knots and the breathing under control tends to help that. Also, never forget that your horse can sense your emotions – if you're nervous, it'll put your horse on edge as well. Remembering that tends to help me calm down. Just know that nervousness is not a weird thing, we all get it! It's okay to be nervous. :)
Personally, I notice that the more I concentrate on my worries the worse it gets. If I were you, I would practice normal breathing, stretch a bit, and the occupy myself so that I didn't have time to think about my nerves. Just remember, the more you show, the easier it will get!
Talk to your horse about what you are nervous about. You and your horse work must work together. If you think something might be challenging just trust your horse. And it's not about winning it's about trying your best. Good luck at your show, and hope this helps! :)
When I get nervous at a horse show i try not to think about the height of the jumps and take deep breaths. If you are nervous about the course then after you have walked it sit alone in a quiet corner and do the course on your mind exactly how you want to do it in reality (correct distance,nice turns ...).For me this works really good I hope that you will have the same results as I did ;)