I agree with alot of what ellygraceee said. Definately shorten your stirrups a couple of holes. Then work on really putting your calf on your horse. Right now it looks like you are just balancing in that position, not riding there. If I was working with you I would put you on the lunge line without stirrups and do lots of work both sitting, posting, and two point at the trot working to develop the strength of your lower leg.
Next, to work on your timing could you set up a gymnastic? One of my favorites is a trot pole to a bounce cross rails to one stride vertical to two stride oxer. When you do this, think about keeping your hips centered over the saddle, sinking down into the saddle, allowing the horse to bring your hips up to your body, closing the hip angle while allowing your hands to go forward into the release. Once you get comfortable with the gymnastic, you can add a gentle turn to another fence or line to keep that feel going.
Also work on flattening the middle of your back. It is sticking out. And I feel for you here. I was taught by a trainer to get down and put your belly down as you are going towards the jump, which is a fun habit my current trainer has had to break. Now that I am sitting up, the same hump in my back appeared. For me, really working on keeping my chest up and waiting on my horse to jump up towards me has helped, both with jumping early and with the middle of my back. Now if someone can fix my shoulders.