Go lighter, don’t lift with your neck and focus on keeping the hand in a position so it looks like you are swinging a hammer. I can’t say for sure if you have a mobility restriction or if you are just doing it wrong. When in doubt, less weight, focus. You need to start in a position where your hand is a little more outside your shoulder. While some of the mobility stuff may be important, it could just be a motor control issue that needs focused practice.