You need to work on stabalizing your lower leg. Without your lower leg being strong and supportive - nothing else will come.
When you aren't allowing your heels to do their job, due to your gripping your knees...because you have no lower leg stabillity.....nothing else will come.
The chain of negative reaction in your form over the fence, occurs when you loose 1 part of your body.
You are ahead of your horses motion.
You are jumping the fence before you even get to the fence.
You have no lower leg stabillity.
You are throwing all of your bodies weight onto your horses forehand
No lower leg stabillity. Results you pinching your knees. You pinching your knees, heels cannot do their job. Your heels cannot do their job - you pinch your knees.
See the cycle? All this causes you to lurch ahead instead of sitting and waiting.
You need to do allot of 2 point work. You need to really strengthen your lower leg and work on opening your knees.
I would put you on the lunge line, with no reins. I would make you really work on seat into legs.
I would make you do allot of grid work, on a horse that is more experienced to aid you instead of holding you back.
I would work on having you learn to ride your horses rhythm, focus on the horse under you instead of what is coming ahead.
I would work on having you ride your horse, not the fence.
Stop looking at the fence. You know it is there, your horse knows it is there. Ride your horses rhythm. Allow the fence to come to you, not you to it.
I would also have you working on allot of flat work, and lunge line work. You need to stabalize yourself first.
This is where you should be:
Leg at girth. Seat low to tack. Horse closes angle. Heels deep, allowing your body weight to sinken into them, to anchor you in your tack.
Allowing horse to do their job, while you stay out of their way.
*my releases are atrocious. My coach tells me I have the typical eventers release....non existant* Big issue of mine.