Before you start jumping, you need to have the concept and physical ability to ride with an educated leg, or grip. You should have an even amount of contact from your thigh, inner knee, and calf.
You are pinching with your knee, which is causing your lower leg to slide back. Go back to flatwork, placing all of your weight in your heel. Your joints need to be elastic and flexible. Especially while jumping, your heel needs to take the weight after your landing. If it is stiff and rigid, or your lower leg has swung back, you land hard on your horse.
You are also too far ahead of your horse. It is one thing to recognize that you are ahead of your horse (which you mentioned) and an entirely different thing to understand why. I can see that not only are you ahead, but you are standing up in your stirrups. Your hip angle is too open.
Your release, or lack thereof, is not effective in helping your horse jump. There is a tiny bit of slack in your reins, and luckily, your horse probably did not get hit in the mouth from your hands.
The idea of jumping is fun, but until your basic position is correct, you should not attempt it. Go back to flatwork, practice the "educated grip" and no stirrup work. You will find your seat and be in motion with your horse the right way.
Work on your two-point, closing your hip angle, weight in your heels, and pressing your center back. Hold your position, several circuits around the arena. Practice at the walk, trot, and canter.
You also need to be thinking about your release. Before you start using an advanced automatic release, either slide your knuckles up your horse's crest (simple crest release) or grab mane to not only balance yourself and not snatch back, but feel the process of it.
Hold off on the jumping until you can fix the things I mentioned. If you learn bad habits now, your horse will resent it and they are hard to correct in the future.
distances are like men. never grab the first one you see; it's never the best one, more will come along.