See that second pic? That one looks very good other than your heel not being down. Your upper body is just enough out of the saddle to free up your horse's back, and you're releasing your horse's mouth. If you can find that position and take it for a spin over low bounces, either cross rails or low verticals spaced 9' apart, that will help you develop a good muscle/position memory.
Before you start, stand straight up in the stirrups and find your balance to where you don't need to hold onto your horse's neck to stand up. Then, without changing anything from the knee down, almost sit down to where your just hovering over the saddle as you are in that second pic. That's a balanced 1/2 seat. Without changing that position, first practice cantering around like that concentrating on keeping your heels down and lower leg quiet, then do the bounces. Those will help you feel the proper jumping rhythm and help keep you from getting left behind or jumping ahead. They're very fun to do too! Your only job is to maintain your position and release his face over the bounces. Good luck.
You just have to see your distance...you don't have to like it.