It looks like you ended up pretty far out on the shoulder (far from the breaking part of the wave) by the time you fell. I think part of why you fell is because of the way you were turning, and the fact that you were losing speed from being so far out on the shoulder. Hard to tell from the video, but it almost looks like the wave was coming to an end?
Either way, before trying to tackle a cutback it looks like you need to be in more control when trying to turn your surfboard. The most immediate thing that stands out to me is that you're not really using your upper body at all when trying to turn your board. I see you're trying to apply pressure to your toes or heels (depending on the direction of your turn) in order to turn the board which is good, but your upper body remains still. Most turns in surfing actually begin with rotating the upper body in the direction you want to go (by turning your shoulders in that direction). Applying pressure to your toes or heels is good, but not without this first step.
So before you try doing a cutback, I would try working on maneuvering your board better. Try to drop in, go down closer to the flats of the wave, then do a bottom turn and come back up closer to the top of the wave. Before you get to the top, begin your motions to turn back down again almost like you are dropping in again. Repeat this up and down turning a few times to try and stay in the pocket. Just make sure you lead each of your turns by rotating your shoulders in the direction you are turning. When you get good at it, you'll find yourself generating too much speed at which point you'll want to do a cutback.
You can read some more about leading your turns with your shoulders here:
http://bit.ly/2g6iq2r
