Anything I need to do goes on a list and I reward myself with some kind of distraction only after any item on the list gets completed.
Usually it's a sign that my mental batteries are running low or even past empty but some stuff still needs to get done. I find that after the third or the fourth item from the list gets completed habit takes over and I'm getting 'in to it' more and more and those molehills-turned-mountains disappear with greater ease.
Good luck combating this, it can be a real problem, especially if it persists for a longer time.
I've found that sometimes it is symptomatic of some underlying issue, something else in life that is deadlocked and needs solving first, not sure if that could be the case with you.