If you post a video of your surfing on the wave, we can offer you targetted suggestions...
While wave tuning is important, I'm 200lbs and I can stay on the little stock Nautique GS23 wave, so if there is a wave back there you can probably stay on it with the right technique (and an efficient board). (The fastest board i've ever ridden is the Hyperlite Automatic... and it's seriously fast compared to most boards.)
I've also taught about 14 brand new wakesurfers in the last two years, from never been on a wakesurf board, to dropping the rope. (and my 8 year old daughter is going to drop the rope any day now)
The key lessons I give people are...
1) Move your feet back more. Almost everyone starts with their feet too far forward, which makes them not have enough "brakes" to slow, which makes them scared when they get close to the boat, which makes them ride the wave all wrong. Your back foot should be touching the rear bump-stop, and your front foot should be positioned so that you can stand balanced when somewhat on the wave with the rope slack. You get speed from shifting your weight, not from moving your feet (we will cover this in #3 below).
If you are constantly "leaning back" and your back leg is getting sore, then your feet are too far forward. Move them back.
2) Move your front-foot towards the toe-edge. Most people ride with their front foot too far towards the heel side, which makes it hard for them to lean into the wave. This is fine when holding the rope, because the rope helps pull you towards the center (even when slack), but when you let go, riders tend to drift out away from the wave. Once the front foot is reasonably centered, or even a little toe-side, it's very easy to turn by turning your shoulders in the direction you want to rotate.
If you find yourself "tip-toeing" to turn into the wave, or falling out the side (not over the back), this is a good sign your foot needs to move towards the toe-side.
3) lean forward and back with your hips, not your shoulders. Most new riders that are falling out of the wave dip their shoulders over forward by bending at the hip. This is not the way to lean forward. You want to straighten your back leg and bend your front leg, doing a sort of "lunge" onto the front of the board. It's a bit like a yoga warrior pose. Another way to think about it is "shifting the board behind you", which is a different way to describe the same motion. If you learn to do this correctly, unless the board is oversize, you should be able to nose-dive before you fall out of the back of the wave. Make sure you practice braking and feel comfortable slowing the board before you try this, because once you learn to put on the gas you will quickly get close to the boat and need to put on the brakes.
If you are drifting "up and over" the back of the wave, you are not getting your weight forward enough. Move your hips and entire body forward by doing a lunge on your front foot.
4) Ride in the direction of the wave, not the direction of the boat. Most new riders tend to start out riding down the wave-face and away from the centerline of the boat (to the right if your goofy, left if you're regular), and then using the rope to pull back into the wave. This is not where the energy of riding is. Think about an ocean surfer inside the wave-pipe.. they ride directly in the direction of the wave. When wakesurfing, this direction is towards the centerline of the boat, *not* the direction the boat is traveling. Once you ride in the direction of the wave, you will get much more speed, and then you'll need that hip-lean and brakes to control your speed.
I hope those tips help. If you can post a video of you surfing, I can probably see what's going and give you a more focused set of suggestsions or "drills" to try and train your body.