I'm with the "it's ok to procrastinate sometimes" camp. Lot's of exercise plans include a "junk day" where you're allowing yourself to eat whatever you want without guilt. The reason is because it's good to reward yourself with something you like, and because it's near impossible to continue at something forever without little breaks.
From the sounds of it, maybe your "junk day" breaks aren't regular enough to keep you balanced, and you've reached an overloaded point.
Take regular breaks, don't feel guilty, and your net productivity will benefit.
If I spend all day browsing around the web unproductively, I don't feel motivated at all.
If I say "You know what, I'm not getting anything done. I'm going to play video games for a few hours.", I feel energized to get work done afterward.
I think it's the fact that browsing aimlessly is like knowing you need to run an errand and then sitting in the car with your hands on the wheel, but not turning the key. It's close enough to what you SHOULD be doing that you feel even worse that you're not getting things done. If I take a break to play a game or read a book or something, it's an explicit transition between activities.
From the sounds of it, maybe your "junk day" breaks aren't regular enough to keep you balanced, and you've reached an overloaded point.
Take regular breaks, don't feel guilty, and your net productivity will benefit.