Ultimately, teaching is all-day acting. You can't even take a bathroom break when you need one, and the current image of teaching as a profession is a day-long ritual of empathy, selflessness and excellence. One's lessons shall always be on point, and one's students always engaged. Responses to poor behavior should be caring and take underlying motivations into account at all times. All standards shall be mastered by all students, and all standards shall be taught in multimodal approaches that value all learners.
This is great except for the little problem that teachers aren't robots. We get sick, we have kids, we have family dramas. Our partners lose their jobs, our children move back home, we're not sure we can afford college for our children, we slept badly last night, we were in a fender-bender on the way to school, the bus was late...
The best you can do is build up credits with your students for the bad times, so that they know your blue mood is passing.
I am sick and my pet is in the hospital with a guarded prognosis. I have had this pet for many years and it has been my companion in very difficult times. What with the coughing and vague lethargy I'm not going to be the nicest lady ever tomorrow (and if the vet calls, you'd better believe that's an emergency call I'm taking).