Take some deep breaths before the show and then just do your thing.
Remember that there is really nothing to be nervous about. No one but you will remember who lost during the show. People will only pay attention to whoever won. If you don't do well in the show, then realize that you need to train a little bit more, and get back on your horse and do it again :)
Try to keep a routine that you do before every show, such as how you groom and exercise the horse. This will make you more comfortable and less nervous before shows. Also, put a lot of effort into how you and your horse look, it will probably make you feel more confident. And if you are jumping, keep a steady rhythm while cantering and your striding will be better. Try singing "row row row your boat" in your head! And ride the course, not the jumps. Just think like you are riding a certain shape and you just happen to go over some jumps. This will make the ride smoother. And remember, this is just for fun! It's all for the experience. Good luck!!!
Nerves are normal and can be good if you don't let them get out of hand. It can keep you sharp. Don't worry about your go so much, it's too late to train so you do the best you can and one thing I like to do is pick one thing I want to do better than I have in the past. If you have any patterns, practice them first with an imaginary horse so you can find your cones, etc Then just remind yourself you are here to do something you like with a bunch of other people that like it and have fun.