First off you have a lovely horse very scopey looking. You need to keep your heels down, lower legs by the girth and on your horse and eyes up. You are gripping with your knee which is causing your upper body to pitch forward and your lower back to round. If your horse were to stop you would end up on your butt. Once you got that I'd advise beginning to use a automatic release it will help with your times in jumpers and you seem to have plenty of control over the use of your upper body you just need to solidify your lower leg. Tying your stirrups to your girth with twine is a safe and easy way to get the feeling of where your lower leg should be. Doing that for a few weeks will help you develop the right muscles.
As the others have said - your legs are too far back your realease is nice, depending on the height of the jump, you only need to fold from the hips, doing this you will stop over jumping your horse :) , your horse looks incredibly talented and you should be proud of her :)