tl;dr: Role play a robot.

Try to block "thinking". Every time you catch yourself "thinking about" instead of doing, stop thinking and do something related to the task. Remove all distractions. Put your phone in another room, close your e-mail and do the smallest thing you can do to move your task forward, don't "think about", just do it, as if tou were a robot. Once you get tired, be satisfied with your progress and relax (release your thinking from prison, Now it is allowed to come in). Practice "I must do this" in other areas of your life as well, without thinking, until you develop grip.