First of all, if you have felt this way, you are not alone. I think that most teachers have had days where it seems like they were not cut out for this kind of job. However, I feel that those feelings are what make us better as teachers. They don't break us. Without these feelings of inadequacy or difficulty, we would never improve or grow as teachers. On those tough days is the time when we take a look at our strategies, our planning, our lessons, and we re-evaluate.
For example, if I have a bad day in class, the students are bored, they don't participate, and they give me that "When will this be over?" look the whole class, I know that I need to make some changes immediately. I need to spend more time thinking about how the students can accomplish a task more than what the students need to accomplish.
I think that we need to allow ourselves to have those difficult days, not that we plan them that way, but we allow them to help us re-adjust, not destroy our self-worth as teachers. Everyone in every profession has off days. Some have off years. Just ask a professional athlete.
This is not a call for mediocrity or laziness, but it is a call to realize that we are humans, and humans don't get it right every time. If you are not failing at all, you are probably not trying anything new, and may not be growing as a teacher. So, don't be afraid of the off days, just dust yourself off and do better the next day. Your students will thank you for it.