Do you do any stretching? If not, try doing some basic hip stretches like the pigeon and the couch stretch. Often times that helps with both depth issues while squatting and lower back pain/discomfort.
Get your head down. This is the most common mistake of all videos posted to this forum, and everyone continues to do it. Get shoes. You are over-arcing your low back. This could be why you have back pain. Just keep it extended, not flexed.
Tried not to over-arch my lower back this time, felt great, first time squatting with no back pain at all.
YOu are leaving your back a little flexible. Clamp down on it using all your back muscles and abs, and lock in in extension. And still, get your head down. You did better, but now instead of locking it in an arch, you are leaving it loose - you can see this at the top.
And you hold your breath with a closed throat / tongue - you don't Louis Armstrong it.