You are bending at the waste when you ride, which puts your weight forward. You will occassionally fall forward when you ride like that (I did for 3 years until they taught me how to ride properly at the wakeboard camp). It's okay to have a bent waste when you are coasting, but you need to get your hips forward before you launch, the earlier the better. To understand the position, stand straight up with your feet the way they would be in your bindings. Now, push your pelvis forward and tighten your glutes. You can roll your knees forward and sink a bit for this exercise, but keep your pelvis forward (when sunk, that is your approach body position). Stand straight up like this and that is the correct launching position at the top of the wake for a progressive WTW. You will be angled back of course relative to the world because you will be on your HS edge, but your body position will be the same.
To practice this position on the water, cut out heelside as far as you can until you can go no further. Then, start leaning back on your HS edge hard, while at the same time thrusting your pelvis forward (which will eventually point straight toward the sky). Straighten your legs (while keeping your pelvis forward) and continue to lean/edge ever harder. Before you know it, the back of your head will be a few inches off the water, you will be pulling the boat, and you will feel the correct body position for probably the first time. Once you have that down, do that on the your HS WTW jumps, just not leaning quite so far back. You will sky and you won't fall out the front.