Answer

By Your Horse

Show Jumping Rider and Trainer Carol Mailer advises:

Obviously, it is the show situation that is causing the problem. What you need to consider is whether it is the actual show, with other horses about and a different atmosphere, or the fact that you are riding her differently. Are you tense and worried, or does she just find a show very exciting and an excuse for not performing as well?
Be very honest with yourself and analyse whether you are the same calm and efficient rider you are at home. Dress her up to practise at home in anything different that you would normally save for shows. If you wear different boots (you or the pony) then practise in those. Try to hire a different venue to take her to and practise being away from home without the show atmosphere. Play a radio when working her, if possible, to get her used to loudspeakers. Try very hard to establish a warming up routine at home, and use this every time you jump. Then, when you use the same routine at a show, it will give you both the confidence to perform as well as normal. If you change your style because you are nervous, then she will change too, and not for the better.
Try to make sure that the only thing that changes when you go to a show is your location.