You’re struggling at the top because you get bent in half at the start and can’t recover. At the point where the bar stops, your back is completely rounded and you don’t have a shot of locking it out. As VTTrainer said this is a very disadvantageous position and your body’s mechanics are working against each other.
StormTheBeach said this in another thread and it’s pretty much the best analogy I’ve heard on the subject:
Your upper back transfers all of the energy that your lower body is producing into the bar. When it gets rounded like that, you’re losing a lot of that energy. For example, what hurts worse, getting hit by a foam noodle or getting hit my a baseball bat? Your back works the same way.
To fix this, you should really do some heavy upper back work. Shrugs, pin pulls might help, pulldowns, rows. Then you should also figure out how to use your legs to drive the bar off the floor.