Think of your basic jumping position as squatting in the saddle, if you lean too far forward, you tip over because your base comes out from under neath of you.
You are really jumping for your horse, when all you need to do is stay in your half seat, letting the horse come to meet you, and releasing with your hands. At this height, you don't need to be moving a quarter of the amount you are.
It may be helpful for you to practice your jumping position and building your strength over poles.
If your horse says no, you either asked the wrong question or asked the question wrong
And God took a handful of southerly wind, blew His breath over it and created the horse