In the first (English) video your stirrups are too long. For general English or Western Pleasure position the bottom of the stirrup just below your ankle bone.
You are inadvertently smacking him in the mouth. That, coupled with the way that he rides "front wheel drive" on his forehand, looks like it's a vicious circle. You try to collect and pull him together, and he doesn't like your uneven feel.
FOR YOUR HORSE:
I suggest gymnastics for him. Start on the ground and lunge him at the walk and trot over poles and small, friendly barrels (plastic ones work well). Don't worry if they barrels roll--that's HIS problem. DO secure the poles bc stepping on one of them could pull a muscle. You can use rocks or bricks OR cavaletti are best bc they roll after
the horse bumps them. (Gotta make some more--mine rotted out. =( )
He will look down before he goes over the obstacles. This is normal. He will also jump them at first, then get lazy and just step over or trot over them. You are looking for a relaxed swing in his gaits. You want to establish a rhythm that you might find a tune to hum or sing--go ahead and sing to him bc he'll enjoy it--NO KIDDING!!
Here is a 2-part article that might help you. Otherwise, talk to people at your stable. Jumper Riding Tips GYMNASTICS FOR EVERY JUMPING HORSE AND RIDER
Yes, I KNOW you're not jumping. The principles are the same for flat work. =D
When you are satisfied with his groundwork, get onboard and repeat JUST the ground pole/cavaletti work. The cavaletti can remain on the lowest setting. Look beyond the cavaletti instead of down and drive forward if he slows down.
As much as possible, ride him on a loose rein. My teacher had us put our knuckles in front on the pommel at the trot. Any time you spend without contact will help you to keep your seat independant of your hands and make you a better rider. You have a very nice horse and I'll bet he responds favorably.
This is HARD WORK for your horse and it will take some time to develop the muscles to collect. Most horses like to travel on their forehand, and some are naturally built a little or a lot downhill so it's VERY difficult for these horses. My old QH, "Ro Go Bar," (1982-2009, RIP) was QH-racehorse blood and built downhill for that purpose. I got him to collect and throw his weight on his hindquarters, but really took some time. He remembered and always flexed after that, so it CAN be done. Keep us undated. =D