Sorry, I don't really see any runaway horse in the video. I see a confused horse, tho. I see a rider leaning against the cantle, bracing with the legs in lope, and see you rather heavily falling back in the saddle while posting. I might be wrong, im watching on the phone, tho.
I would, first and foremost, do, as was said, more groundwork. Then doing way more walking first to warm her up. I would NEVER go straight, do lots of circles, making them smaller, then larger again, with leg and weight only. Then I'd start trotting, first nicely forward, again, lots of bending, changing hands. Try to get up out of your knee, when posting, don't stand straight up, rather just lean a little forward, and don't heavily sit down either, just lightly touch the saddle and get up again, so you stay WITH her movement.
I would lope her at the end for a couple of laps only, again, circle, paying attention to the right lead!
All the bending will get her to engage her hindquarters, bring up her back, get more comfy to sit.
Do every change of direction with your weight and legs FIRST, forget the reins.
Find the deepest spot in the saddle by getting out of the stirrups and stretching your legs down...it will pull you right in it( and the stirrups will seem too short after lol).