I absolutely love your horse in this picture:
I LOVE how he is tracking up, I love how he is rounded and reaching down for contact and how he is rounding his back.
Anyways - with your jumping form -
Fixing your lower leg, firstly, you need to realize that you have to be around your horse, not ontop. You are clearly more comfortable on the flat, and jumping is an issue, it is with me - but what I find helps the most is
STOP RIDING THE FENCE!
When we focus on the fence, we stop focusing on our horse. We cannot jump the fence, that isn't our job, that is our horses job. When you stare at the fence, you start riding the fence and we cannot do that.
Learn to focus on your horse. Remember, jumping is dressage with speed bumps. So you wouldn't be riding the letters ahead of you, you'd be riding your horse to those letters. You allow those letters to come to you, instead of you racing to the letters *speaking dressage now* so why would it be any different when approaching a fence?
You cannot ride to the fence, you have to allow the fence to come to you.
Ride your horse, your partner, the 1100lb animal with a mind of it's own, that is under you. Remember him? Lol. What I suggest is lunge line work, with no reins - works wonders and is an amazing tool to have. Lunge Line work with no reins, makes you focus on your body position to influence your horse, and makes you focus on your horses rhythm and body to influence your position.
Learn to ride your horses rhythm. Learn to remain over his center of gravity, and learn to remain with your horse and work together in unison.
GP Jumpers do this, so why can't we? Olympic Riders do this, so why can't we?
When you've achieved riding your horse on the flat with no reins, start incorporating fences - cavaletti's, xrails - and you would be surprised with how you change your form in the saddle, to remain with your horse.
Now that we worked on your mental state when riding and your primary focus - we can address your body.
1) Stop gripping with those knee's. Open them up, your knees are not supposed to be supporting you, your lower leg and heels are.
2) It is fine and dandy to shove your heels down, but why are you doing this? What is the purpose? It isn't about shoving your heels down, it is about allowing your bodies weight to flow and dispurse naturally into them. They are your anchor, your saving grace. But you cannot do this, if you are gripping your knees and blocking that flow of weight from happening.
Remember - every body part is like domino's, or a chain. Without one, the other cannot be effective.
3) You must be wrapped around your horse, not ontop of your horse. This comes with opening your knees and allowing your weight to flow into your heels.
Your legs must be at the girth. Absolutely must be. Not under you, not infront of you, but at your girth. Your toes should not surpass your girth, if your toes go past -they are too far ahead. Your toes should be just at the center of your girths width.
Your inner leg should be touching your horses side. Open your knee's up and wrap your lower leg around your horse, as though you are hugging him with them.
4) Activate your seat. Riding isn't leg into hands - it is seat into legs, hands come last - to fix your powerful horse, you must bring him down through your seat. That will help you with your tense upper body as well - do you take dressage lessons?
5) Ride your horses rhythm, not the fence. Sit, get your 3 points into the saddle, soften your lower back and ride his rhythm. Slow your seat, bring him down to you, lift him up with your upper body and your hands. Relax, breath out and allow the fence to come to you, not you to it.
Ride your horse.