Just reposting this because IDK how to fix it. I guess stick my ass out more? As soon as I throw weight on my back I can't keep my core stiff. I fall apart. I drop down like an asshole. This is so frustrating.
Yeah, that's significant rounding and not safe. Let's fix it!
First off, slow your descent. You're diving into the bottom, which will cause you to loosen up your hamstrings and round your back. A measured, controlled pace is better.
Second, do a squat stretch before you do your reps. Get in the squat position, arch your lower back and stretch out your hips, hamstrings and adductors. Really shove your hips back and rotate your pelvis forward. Make sure your knees are wide enough apart to fit your torso when you're at the bottom (hard to see from this vid).
Then you'll need practice. Go slowly down with a tight upper and lower back, feel the hamstring tension from being stretched by your back pulling your hips forward. In the hole, you should feel a 'bounce' off your hamstrings and adductors as they get even more stretched while you deliberately force your lower back to arch. You're not sitting 'down' between your legs, you're sitting 'back', as if a chair is just slightly too far behind you.
Try that a couple of times then film yourself again and get back to us! Good luck, you'll be squatting stacks of iron with perfect form in no time.