Maybe school the day before? If she/he spooks then don't pull on the reins and don't tense up. I've never really had that experience but I do know it's scary. Try and sit back and pat the horse at the same time if it's possible and you know you can sit the spook.

I have a mare who can be reactive at times too. When we show, I try to find a time with either no one in the ring or very few people(usually very early in the morning or in the evening) and walk them by hand. I ground work + lunge and make it easy and calm for them to be there. If it is super busy, I'd atleast hand walk around the whole ring if possible

Definitely warmup day of, leave PLENTY of time to do this so you can flat your horse down if super energetic/looky. If you're allowed in the actual show ring, consider hand-walking your horse around to allow them to look at everything and get used to it. Even better, ride in the ring if you're allowed. During the show, earplugs are a great tool to help focus your horse as long as they don't want to spend all their time trying to shake them out. One thing I like to use to help my horse keep his calm is Perfect Prep. I like the extreme formula that only needs 90 minutes to work and lasts like 6-8 hours. It generally helps to calm the horse. You can find it at just about any tack store (if there's a saddlery at the show and you don't have time to buy any before leaving, the store at the show will more than likely have it) or you can check your local co-op store too, if you have one. Bring along some cookies to feed to your horse while you hurry up and wait, that seems to help anxiety in some too. Finally, relax! The horse will be able to tell if you're anxious, and that will make them anxious. Take a deep breath, relax your muscles, and have fun. You got this! Good luck!

And if your horse bolts again, try the pulley rein stop. This is a fantastic emergency stop, but generally only used in extreme situations because it can be quite forceful. Here's how you do it, and I'll link an article and video too!

Sit in your saddle, and let your feet come forward just a bit ("feet on the floorboard"). Shorten your reins! Brace one hand (I prefer my nondominant) on the neck. With your other hand, give a few short, sharp tugs that go back and up. Most of all, remain calm (hard to do, I know. Been there quite a few times myself.). If this doesn't work, try to circle as small as you can, as circles force the horse to slow down in order to maintain balance.

@Castle ~ I'm surprised I hadn't heard of this one sooner considering I watch Horse Class a lot xD

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But yeah, this is super handy, I need to try it next time my feisty pony(who thinks she's a racehorse>>) gets out of hand :D

All of my mares have been trained to respect this stop, it's truly a lifesaver when used correctly! I'd suggest working on one rein bending at the standstill and then work up to faster speeds.