We all do from time to time. I'm going through a big bout of it right now...and I like to read through Diane Hopkins' thoughts because I think she's an amazing (and refreshingly honest) mother. I've never met her or spoke to her through any form of communication, but she is a kindred spirit to me.
One of my favorite articles that she's written: Don't Serve From an Empty Platter
A list of some others that are equally great at buoying me up on down days: Home School Helps (not just for homeschoolers, in my opinion; for all mothers)