You need to commit to leaning over at the outset of the movement. You are afraid to do so until the physics of the situation forces you to do so at the bottom. Your failure ("fail" is a verb) in the second video was because you never even tried to lean over and you just basically sat back down with your back vertical. I'm thinking, also, the bar is a little too high on your back.
Your knees are caving in, which will require you to shove them out and keep them shoved out. Proper back angle through the movement will also help with this.
Good work for the mechanical problems, nonetheless.
I'll surrender the balance of my time to Dr. Arnold.