She obviously can feel the excitment and tension in the air and it probably instantly makes her think she's back at the races.  My horse is an ex-trotter and reacted exactly the same way.  I went back to basics and took him to shows for the experience, I didn't enter any classes, just rode him around the showground to get him used to the noise and excitement.  Once he started settling down I then entered some in-hand classes and eventually ridden.  This worked with him and now going to shows isn't something I dread, but can now enjoy.  The last thing you want is for her to do something that knocks your confidence.  Taking things slowly and in a controlled manner is the best option.  If you decide to go along this route, please speak to the show secretary beforehand to check that they are happy for you to attend purely to acclimatise her and not to enter.  Good luck.