Looking good! But your quasi-instructor is right; that's a pretty wimpy power-stroke.
Try to move your hands closer to the middle of the bar. In the video, your left hand is preventing you from making a tight turn at the top of the window to dive the kite. Similarly, before falling it looks like you were trying to make a small correction to turn the kite up off the water a bit, but accidentally sent it straight up. Putting your hands in the middle will help prevent over corrections like this.
In the same vein, for a few sessions consider moving your rear-line attachment points one spot forward to increase bar pressure if your kite has the option. Like moving your hands to the middle of the bar, it reduces your leverage and increases the force required to turn the kite--which is a good thing when you're learning; you don't want every little unintended movement translated into a turn on the kite. Deliberate actions are the order of the day. Your forearms will complain, but you'll have a better feeling for your kite, even when you switch back to the lighter setting.
Tautologies has some good advice on going upwind. Coming from a sailing and windsurfing background, my instinct was to point as high upwind as possible as soon as I got up on plane. I'm much better now about building board speed first; but I still catch myself losing speed when I try to point too far to windward. And that's trick to going upwind: finding the balance between pointing to windward to control your speed and bearing off to maintain it.
The next time you're in a situation where you're thinking, "HOLY SHIT, I'M OVERPOWERED AND GOING WAY TOO FAST!!!" instead of turning the kite to the zenith to lose speed and get back into control, let the bar out, drop the kite closer to the water, and edge hard upwind. You'll be going scary fast for the first few seconds, but soon your speed will taper-off and you'll be able to raise the kite, sheet-in and reduce your edge pressure. At this point, change direction, and marvel at your upwind progress.