ok. First off, I see a LOT of improvement. A lot. you've done a good job so far in working on your seat, so kudos to you. Flash is a nice horse, and the better you get, the nicer he'll look as he starts to feel better able to lower his head and lift up his back.
But, that depends on him having a good saddle fit. This is something you want to truly verify in any case where a horse runs with his head up and a bit of a braced, hollow back, as does Flash.
The other thing is for you to get a bit quieter in your sitting on him. You really did better at the canter than the trot, and that canter was WORLDS better than the one you posted before.
It think that one of the things is that you are rolled just a wee bit too far forward on your "pee-pee". IT's called a "fork seat". Things go ok as long as the horse doesn't make any sudden changes of speed or direction. But it means that you can't quite absorb movement as well and you don't have the ability to make the horse come to your seat (i.e, control his speed and energy with your seat and core muscles).
AND , your stirrups are too long. your are riding with your toe pointed downward, so-called "fishing for your stirrups". While doing this, you get rigid in your knee and your whole lower leg comes off the horse, putting you even more in the fork position. It is possible that the saddle is too wide in the front, thus it drops down over his withers, giving it a downward slant that puts you on your fork AND makes the top of the saddle bump the top of his wither bones and/or gouges into his shoulders, (thus the hollowed out back).
your upper body is , again, much better than before.
Thankyou for sharing these videos. one day you will look back and say, "I rode like that?!"