If she's an ottb, jig jogging is her responce to exciting environments. Instead of manking her walk, try pushing her forward into trot. Do lots of circles, change the rein, Shoulder in (if she knows it). Keep her guessing and focusing on you.
When she jogs, do you take your leg off? Try keeping your leg on lightly - my gelding used to be an absolute prat in dressage tests because as soon as I would touch him, he would shoot off. I now ride with more leg than I used to and I can get a pretty good test out of him.
My instructor likens it to giving your horse a big hug, saying "its ok buddy, I am here for you, I am with you, we will be fine." Sounds strange, I thought he was nuts but hey it works with both my boys, as soon as they get nervey, leg on, ride them forward and they settle.
Don't give her too big a circle to work on. 15-20m should be fine.
Is there a loud speaker at the events? My OTTB used to get worked up by the loud speaker - it reminded him of his racing days... I took him to a big event and left him in the float yards, right next to the cross county course near a loud speaker so he could hear the commentary. :) It took him an hour or so or running like an idiot, but he eventually settled down and the next time he was at a show with a loud speaker, there was no jig jogging or bad behaviour.
Before chilling out, scary 3* horses in the distance, loud speaker behind him.. Terrifying. bobrook.jpg
2 hours later, after a walk around to the XC warm up, investigating SJ warm up areas and watching about 20 horses go past him on XC. bo.jpg
Do you get nervous and stressed when taking her out? She will be picking up on that too. Get a good night's sleep before the outing. Maybe ask you mum and dad if you could take St Johns Wort which will help with stress levels? Blackmores make a good suppliment which you can get from the chemist. St John's Wort - Mood & Relaxation Support - Blackmores
Also Rescue Remedy can help with fear, anxiousness, nerves and all those associate feelings. I take it myself and also give it to my horses. you can get it from the chemist too :) Rescue Remedy | Martin and Pleasance | Rescue Sleep, Rescue Cream, Rescue Pastilles, Rescue Chewing Gum. | Martin & Pleasance
Good luck, let us know how you go :)