What works for me: admit that you are going to want to do other things. They are going to show up even as side comments in the material you are learning. Keep a list of things you will work on later. Don't sort it, but make a deal with your future self to review the list after you reach "the end" of your current study. "The end" should also be clearly defined, hopefull by some small capstone project. Amorphous goals are never attained, and every topic is endless. Set an achievable goal.