I wasn't able to watch the video.... since I couldn't see what's really going on I have a few guesses. My main hunch is that you aren't using your outside aids properly (particularly your leg) and that your horse isn't balanced enough. When you land and look for your next fence, you can't turn by pulling on your inside rein. You must use lots of outside leg, and block the shoulder with your outside rein. The inside rein simply opens (not past his shoulder) to give your horse a place to go. When you're working your horse on the flat you want him to carry his weight on his haunch instead of on his forehand, so lots of lateral stuff will help teach him to do that. Shoulder-in, counter canter, turn on the haunch, etc. Don't think about it as being a matter of striding, just stay on a proper pace and keep your horse straight and balanced and the roll backs will start to happen.
But like I said, I couldn't watch the video so that's just my guess of what's happening. :) Post it again and I"ll take a look!