I think you would benefit from an extra second or two of setup before each rep. You are not that far off but it looks like you lift your chest right into the deadlift. Your back is slightly rounded at the start of the pull.
Try gripping the bar, bending down and then squeeze your chest up like your nipples are lasers and you want to point them at the wall infront of you. At this point your lower back should get tight and you should focus hard on keeping it flat and contracted.
Now that you are in position with your chest up and back flat you can start pulling with a strong focus on keeping the lower back tight and flat.