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.
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.