Most of us have ups and downs. They're part of life, so it doesn't really make sense to beat ourselves up about it.
I was talking today with someone who was angry at herself for not getting enough done in the last three days. She had, by her own admission, been very productive in the four days preceding--a time in which she wrote a complete 20-page submission to be a chapter in a book.
Now, admittedly, she is trying to finish her dissertation and working on that chapter wasn't working on her dissertation, so it was an obstacle. But productivity is generally a good thing. Her dissertation isn't on any deadline that is about to fall.
Anyway, my feeling was that, after working extra hard for a few days it's neither surprising nor inappropriate to have a few days in which it is harder to get going. It seems to me a natural enough response to doing something that is draining.
It's nice to maintain an even keel most of the time, and to make consistent progress on a regular basis. But to deny that there are ups and downs, and to blame yourself for the downs sets up a pattern of negative reinforcement that probably isn't going to help you get your work done.
We have ups and downs. If we can accept and choose good responses to our ups and downs, then we'll find it easier to maintain our levels of productivity because we won't be getting weighed down by regrets for what we think we should have done.