I've had the exact same problem and now I'm able to set my back much better. Some recommendations:
- film your deadlifts, every single rep, including warmups. this way you see whether your back is rounded. if you don't see it you can never correct it
- Stick to the blue book 5-step setup. In step 4 (setting your back), make sure your back (including lower back) is flat; and keep it flat as you start pulling
- if you can't keep your back flat BEFORE you start pulling, it might be a flexibility issue. I'm not sure how to solve this (perhaps stretching? don't know). But doesn't look like this is the case from your video
- if your back rounds as you start pulling (I had this problem), it's probably some combination of two issues 1) a mental block, you just have to practice and overcome it; 2) your erectors/hamstrings are not strong enough. I think for most people, pulling on a rounded back produces better leverage which makes the deadlift easier. So keeping your back flat would make your leverage a little worse, requiring stronger muscles. So the solution is simply to lower weight until you can pull it with perfect form and go from there
- when I had to change my form and keep my back flat, deadlift actually felt harder, less comfortable and weight had to go down. You just have to get used to it, especially the discomfort. The correct starting position of a deadlift is just not comfortable.
- search for starting strength deadlift on YouTube and watch ALL the videos that come up. there should be at least 5. There are a couple learn to deadlift ones. plus 3-5 focusing on setting your back and other common problems. I watched each of them at least 5 times.
this was a tough nut to crack for me so I hope the above helps.