It seems like you are not the boss of your horse. I would not try to go in a halter class unless my horse respected me and listened to me and had manners on the ground. Even worse under saddle. If you slapped your horse on the rump, what do you expect? She will run forward or even kick out. It seems as of she needs a little more training on the ground. She needs to respect you. She went for the grass because she knew that she was going to get away with it. Next time she does that, click her up and oull her head up and keep walking. Don't giver her time to go for the grass, be prepared. Walk confidently past the grass and be preapered for her to snatch at it.
But good luck on your show and I hope to see some pictures