At first, you have to be calm, do the same work as at home. Dont be nervous, relax. But also you have to be a little bit confident!
Eq confidence is a good calmer for competition horses and used a lot for eventing. Always use the same warm up in the collecting ring. Stay away from the other horses incase they wind her up and stay calm and relaxed yourself. The more she goes out and over time she should get better.
School a ton before the show! and mare magic is a natural supplement
Use this herd called chamomile flowers and it is a natural LEGAL calming agent and it works really well but it does make your horse pee a lot
Try to warm them up by going for a long walk or trot or canter around the area To get them used to it
maybe try groundwork to get them focusing on you. helped with my tb
I usually stand next to them for a while after I warm them up. Or I get there a day early and warm them up and show them around, I've found these have helped
Try lunge them before, it often helps!
Try to do more trail rides :-)
Naf’s Magic calmer was good on my old pony x
Relax and try to ride as you would at home, usually your horse just feels that you're stressed about the show and starts to tense about of it. If you are relaxed, but your horse is still too energetic, it means you didn't work enough with him the day before the competition. So in that case, you can try to ride him more on the day before the show or lunge him before the warming up.