Like others said, your lower leg has slipped back. If you want an idea of where it should be, next time you're on that horse, stand straight up in the stirrups like your were standing on the ground. Find your balance to where you do not have to hold your horse's neck to balance. Then, without changing your lower leg position, sit almost down in the saddle to where you are just hovering over it. That's the correct jumping position. You look like a nice match for your horse. I'm sure you'll do great.
You just have to see your distance...you don't have to like it.