I deal with this quite a bit, and it is hard when you hit the wall, seemingly over so little. One thing that really helps me is being on a consistent sleep schedule. Also, taking mini breaks throughout the day that include no use of my mind whatsoever, i.e. a brief walk, a quick shower, tea or coffee on the porch in the sunlight... absolutely no reading, computer, tv, people, or other stimulation. Meditation helps me, too.