your starting position is really rough. It looks to me like you have very weak hips, glutes, and hamstrings. You are starting your pull from a position where your hips are WAY too low, and your back is already rounded and in very bad position. As a result, you have zero tension in your hamstrings at the start of the pull. Basically, you are trying to squat the weight up using your quads, only your legs are too weak so your hips shoot all the way up until your back can take over completely. The weak hips are also the reason it's so hard for you to lock the rep out - your butt simply isn't strong enough to squeeze your hips through, and your super rounded back is putting you in a terrible position to lock out the rep.
In my opinion, your deadlift needs a total re-boot. First of all, you need to work on your mobility, so that you can get into a starting position with a flat back. Look up a thousand videos of people deadlifting with a flat back and videotape yourself until your setup looks textbook. Obviously every lifter needs to develop their own style, but at this point I think you need to start with textbook technique, and as you grow as a lifter you can change things if you need to. Second of all, you need to learn how to use your hips. you aren't hinging at all, you are pretty much just standing the weight up using your back. Look up "good mornings" and work on learning how to do those with perfect form for a thousand reps. It will teach you how to set tension in your hamstrings, how to keep your back flat, and how to squeeze your butt to get your hips through. Once you learn how to hinge, again work on your setup, being sure to set tension in your hamstrings before the pull. And finally, keep the weight light. I'm talking REALLY light. Once you get the hang of pulling with a flat back, from a better start position, using your posterior chain, you need to do tons and tons of reps, with perfect technique, to cement that form in your head - and the only way to have perfect technique is to go nowhere even CLOSE to heavy for a little while. Idk what you are pulling in the video, but I would say take around 50% of that and start there, doing speed pulls. tons and tons of speed pulls with perfect form. Maybe change the weight from week to week : do 50%, 55%, 60%, then back to 50% for 9 weeks or so. you need to INGRAIN that movement pattern into your mind and your body. It's goign to be a real hit to the ego, and not fun, but if you do those things I think you will come out on the other side with beautiful technique, and THEN you can start to build up some serious strength
Best of luck, and keep at it.