Are you saying you look like this?:
The rider in this picture's stirrups are way too long(her knee angle is to wide) and she's jumped way ahead of the horse without releasing his head.
You need to close your hip angle to getter ballance yourself over the horse. Often times when you stand up like that you're jumping ahaid of your horse, which is bad because if he refuses you're going to end up on the ground. Also, it usually means you're not driving him enough, which leads to him falling on his forehand or front end instead on using his back to propell him over. Give the horse lots of leg before at at the jump and don't jump ahead.
Let the horse's purpulsion lift you up. You also do not want to "duck" or throw yourself down on the horse's neck. I reccomend the book
Hunter Seat Equitation by George Morris.
This girl's hip angle is very closed, she's ducking on her pony, who is jumping the oxer wonderfly and she is doing a good job staying off of him for it but it would most likley be looked down upon if it were she who was being judged and not the horse.

This rider's leg has slipped way back, causing her to fall forward too far onto her horse's neck. When you don't have good, strog legs that work independently of the hands, everything is much harder to accomplish.

This rider has very good equitation, see how she's pushed her butt back and closed up her hip angle, keeping ballance over the horse's back over the fence. Her legs are underneath her driving her horse and giving her stability.
Hope I helped and pm me if you want to talk more about riding.

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