YOu can keep the video, I'll take your uber cute horse!
hard to see, so hard to make much of a crit.
most obvious is you need to get your elbows back against your rib cage. If you need shorter reins, don't do it by pulling your hands back toward your tummy, and thus putting your elbows out "chicken winged" . shorten the rein to where it should be, and make minor lengthen /shorten witih your hand.
Use more core to prepare your horse for halt, DONT ever snap the reins upward, as you appeared to do in the very last downward transition. use core, "HOLD" him, a bit of squeeze with your hand, then breathe him down into a walk
I wish I could see better to say more, but you've got a decent start, no mistake about that.