If bad thoughts hit you, never stew on them, KEEP ON THE MOVE.
No matter what. Keep moving, if you can't go back to sleep, get up, move about the house, do something else and then try to go back to sleep shortly after.
If there is something small you can do, do it. do NOT stand still and stew in your thoughts, "but I can't do it", baloney, there is ALWAYS something small that you can do at any given moment, be it taking a walk, taking a shower, doing some cleaning, etc. Don't listen to the stubborn brain.
If i am fatigued I would take a small nap, if I feel stressed I would either work out or take a shower. In life we have to have an arsenal of responses to our negative behavior at our disposal so that we can learn to conquer ourselves, each time the brooding begins think of it as an opportunity to master yourself and the stubborn brain that we all have. It is a very very stubborn thing.
But at the same time, take things easy, it's okay to make small baby-steps, which is why I say if you feel like you can't do anything, try to force yourself to do something small, as it helps you build momentum to get things really going again and get your thoughts off the negativity, remember that we become what we think, so have good thoughts.
Just take it easy and relax in your smallness, you're not going to get everything done at once, in fact you don't need to, moving slowly and steady with a patient heart is a great character trait, so just take it easy and find somethign to captivate your thoughts, get a jigsaw puzzle, solve cross-word puzzles, etc. Reclaim your brain, but whatever you do, do NOT stand still and stew, as that ALWAYS turns into bigger problems and character flaws down the road. Nip this thing in the bud!