That last video looks pretty good.
Your feet are rotated waaaaay out, probably a left over from riding in western saddles. To get your hips and legs sitting closer to the horse, try stretching like so: once you get on the horse, drop one stirrup and bring that leg back up so you can grab your ankle. Then pull up and back as far and for as long as you can stand it. Switch sides, and then do that two or three more times.
In not too long you'll have to drop your stirrups a hole or two because your legs are draping longer over the horse and your thighs will be rotating inwards so your toes won't be pointed out so much.
I'm a strong believer in adults choosing how to live their lives, but I will strongly recommend a helmet. Your call though.