This happens to me all the time, and I find that banning myself from the semi-productive or unproductive stuff just puts me even further down into uselessness.

I once caught myself laying on my couch, counting the stitches on the sleeve of my t-shirt because I'd promised myself that I wouldn't do anything else until I'd finished the thing I was procrastinating over. I recently started playing World of Warcraft again when I realized that, after deciding to take a break from the game, all the time I had previously spent on the game was not going into something more productive, but towards reading increasingly worthless fanfiction and unnecessary snacking.

When there's just one thing on the to-do list that's blocking you up like that, it is really just a matter of noticing what's going on and realizing the effective urgency of the thing being procrastinated over. I think it's actually harder when there are several not-actually-unpleasant-but-equally-appealing things on one's to-do list. I still have trouble with that. Several equally-productive, equally-urgent things I want or need to do, and I find myself doing none of those things, and banning myself from really-nonproductive time-killers doesn't seem to help; I just start counting threads, so to speak.

I think maybe I have trouble establishing subjective priorities when my objective priorities are not obvious. I usually don't have a problem with procrastination (anymore) when I feel sure of any one task or project being more important than the other things that are competing for a given chunk of my time. But that's just a guess, and if correct I'm still not sure where the line between that and my general (extreme)reluctance to start anything that might be interrupted before being finished, is.