Practice without irons. In fact, ride without them a lot. Once keeping your leg quiet becomes more natural, you should not have a problem. Also, depending on the height of the fence, your leg will need to move back a bit, so you stay on top of your horse. Sort of like a counterweight, although I have noticed that nowadays, people do not seem to move their upper bodies with the horse’s center of gravity as we did way back I won’t say when. We had a chute, and had to jump through it with no reins and no irons until it was second nature (and sometimes, with no saddle).
It is never the horse's fault