Two things:
1; Your leg gets tired. I mean, duh. But if you can make that happen later on towards the end, all the better

2; You tuck your head down - Not your chin, your whole head. Its from tightening up from the upper back. Sit bolt upright in your chair, then lean forward and down a bit. Feel the part of your upper back that rotates? Try to limit that a bit. Your head can move, but the back bending in doesnt exactly help.
3; Take a very short step with your lead leg first. You might be doing that already, but its hard to see with the camera moving around and the ground all looking the same
PS:
Your kick itself is fine. Its a Thai Kick, not a TKD Kick. Different body mechanics, different technique. Dont worry about that.
If you try to slow down, your technique might improve, but people like me, for example, learn better by only slowing down to correct one thing, spending 10 minutes on it, then cutting back to full out. Its personal preference.