"So, now I'm gonna write and I won't go on Facebook, or Youtube, or..."
And thirty minutes later you still haven't even opened your file, or took out your pen and paper, or even your typewriter (although, if you'd use one, you probably won't use the PC.)
Problems like these happen all the time, regardless of what you're doing.
Why are they happening? Perhaps you're a lazy git who just doesn't want to. Perhaps you're afraid to discover. Or maybe you're in a great position at a video game and you want to beat it. Or you like to eat and...
Well, you get it. There's isn't a specific answer, because not everybody is hardwired the same.
The only answer is to write down (uh-huh) why exactly you're doing what you're doing then and see it for yourself.
Maybe you can't change on the spot, maybe not the day after, but change will happen.
And when it'll happen, you'll be smiling.