Be merciless about it. I have an app called selfcontrol (mac). It lets you blacklist websites on your computer for a given period of time. I would recommend doing 6 hour blocks at a min. I actually have mine set to blacklist fb/reddit for two weeks.

Also, next time you begin a project just sit for a minute or two. Try to clear your mind of outside worries or expectations for the work. It generally helps with focusing on the task at hand.