Lower the weight a bit, and make sure you keep your back arched HARD and TIGHT.
Soon, the tight arch will feel very natural even when the weights get higher, and you will be able to hold it.
I often thought I did everything with good form... until I videotaped myself and posted on this board...lol. Maybe you can find someone to take a look at your deads. Maybe someone in your gym competed as a powerlifter, etc.
...make sure you keep your back arched HARD and TIGHT.
Soon, the tight arch will feel very natural even when the weights get higher, and you will be able to hold it.