I've had a lot of success with the pomodoro technique. You start with a todo list for the day and then do work in 25 minute chunks with 5 minutes off in between each chunk. I've found that giving myself those 5 minutes of time to deliberately not work makes it much easier to actually focus for the 25 minutes when I am working. It also forces you to look at what you're doing every 25 minutes so you don't head of down rabbit holes.