I wrote a really long reply but my POS computer dropped it. I will try again, but maybe a truncated version.
Basically, try doing more of this at a walk first. I agree that you seem to have unsteady rein contact. At a walk you will be better able to follow the head and use the rythm of the walk to invite the horse to reach downward.
I can see you "fiddling" with your reins a lot. I know you need to invite the horse to reach down, but if you don't have enough contact with the mouth to be able to follow it, you will miss the place where the horse starts to go down, where you would follow softly so that she knows "yes, that's it"
The think you want is for the horse to both reach downward AND reach outward with an arch in the neck. You don't want her to come behind the bit, at all , not behind the vertical, not at all. Think more of her neck reaching forward in the way a horse will arch and reach forward to touch noses with a new hrose (the way they are excited).
Secondly, you wan the horse to step through with some push from behind. At a walk you can really feel the push . You visualize the horse kind of pushing through the "gate" you form with your reins and the bit.
Don't let your arms brace out at the elbow. Otherwise, you ride quite well and you two will be a wonderful pair.
Lastly. for such ayoung horse, give her a loose rein stretchdown a lot and mix it up to relieve boredom.