The first advice I would have for you is to make sure you train well before the show. Once your at the competition you don't have to train, just make your horse relaxed and warmed up enough.
After the show, have a debriefing with your trainer to understand what was good and what was less good. So you know what to train on until the next show. I suggest you to film your courses so you can also see it.
My last advice would be to focus on your ride and your horse and how you can improve yourselves instead of focusing on the competition which would give you an extra pressure.
And don't forget that it is okay to fail sometimes, you can't always be perfect and neither horses :)