Take lots of deep breathes and say that you are going to be fine and that you are going to do great!
Drink a lot of water and just focus on you and your horse not the other competitors
Try thinking of everything that could go well... and that you trust your horse and you only have to do nothing different than what you do at home(:
I get really stressed also because i used to have a pony that stopped all the time, but i find that rescue remedy works really well (i use the drops)
I also get super anxious before shows, and what helps me a lot is visualise how i want the show to go. As a jumper i visualise the course in my mind and then every curve and every jump i try to get a feel of how i want it to go, or feel like. Also talking to my mom, who is always with me really helps, and making sure me and my horse are warmed up really well (by riding a lot of trasitions) so that i know that nothing can go wrong, and the horse cant get hurt. Lastly i set little goals for myself, so that i am not dissapointed if i dont win the class. For example i used to have huge problems that my horse would stop at the many fences, so i put the goal for myself, not to win my classes or to go faultless but that my horse would not stop at any fences, and then when this happened even though i did not win or go faultless i had achieved my goal and that gave me a little victory. Good luck at your next show!
Keep smiling and never stop, this over faces you looking scared :)
Highly recommend checking out the Oilystable
Take a lot of deep breathes and don't over think anything. Talking to barn friends help me get a lot of stress out. You could also make yourself busy by helping other people, organizing stuff or grooming your horse. Good luck!