You just need a bigger box. That looks a bit dangerous. I find it best to have a myself close to the box (catty corner) but the box is big enough that I can sit back. That way I have no way of missing it and I don't have to "aim" for it.
You doing the classic thing that most everyone seems to do when they sit on the box. Very easily remedied. Watch the vid and see how when you set down you rock back. You get a bit of rounding in the lumbar and your tightness leaves. Basically, don't do that
Keep the arch set and tight. Keep the hips and glutes and everything set. Don't allow any backward motion.
The reasons for this are 1. To protect you back. When you have that load on your back the compressive forces are magnified very greatly passed the actual weight on the barbell. They are simply huge. When you allow any lower back flexion the forces compressive forces actually go up plus the shear forces.
2. You ability to fire the hip muscles are WAY better when you keep it tight. Coming out of the whole from that rounded back and relaxed position is greatly diminishing the effectiveness of it as well as the weight you can move. It also causes you to have to slightly goodmoring the weight. You have to resest the center of gravity before you can move the weight, basically ruining the economy of motion. The best case scenario is that you push you shoulders straight up into the bar and fire the hips forward and you rise up without giving up a lot of forward motion (for the most part...heavy weights are sometimes no man's land).
Most people probably do this and I expected you would.
You're doing a good job coming down on the box slowly.
What's the belt for?