I second these two. If you need ace just to get your horse out there, then maybe you need to rethink his future. We aced a mare to get her to load when we were out of state. It was a mare who lived in a pasture all her life (12) and had only been haltered for the first time a week before we bought her. We have also used ace for medical reasons. But if your going to be riding the horse then its very dangerous and typically a bad idea. Since this is your first time riding at a show its natural for a horse to react like that. Take your horse to shows and try just walking him around on the ground. At home ride with other horses. Try and set up an "in house" show with your trainer. My trainer sets up group lessons for show riding, to get peoples horses used to the show environment in a setting they are used too. Also horses how to handle riding in a show ring with other horses and people around. The night before a show ride your horse and ride him longer than normal. Really really work him out. At a show he will be pumping with adrenaline, and its okay to wear him out the night before if your trying to get him used to shows not win them etc. Now once he gets over all these fears wearing him out the night before will not be to your advantage, as the adrenaline high wont be there and all he will be is worn out. When you get there take him to a big round pen and let him run off some extra adrenaline and just calm down a bit. Just ride around the show grounds and get him used to everything and if he is calm enough try some more complicated things in the warm up ring and if it goes well then enter a class in the show ring. I know there are also other options besides Ace out there. A lot of feed stores will sell stuff like B-Calm (I think its what its called?). I am pretty sure this stuff is not legal for tested shows and I have never used it but I believe (not sure, never used it myself) its not as strong or dangerous as Ace.