Practically every morning I feel a low-grade dread going to work. I love my job, there's nothing else I'd rather be doing. I think it's honorable work that betters society. The kids are funny for the most part, and I get at least one laugh because of them daily. It's rush rush rush all day, but that makes the day go by faster.
So. I guess it boils down to 2 things. One, I never think I'm quite doing the best I possibly could. It's always, "oh, you could have taught it that way", "ooh, that's more engaging and grabs their attention and makes things stick", "ach, you could have handled THAT better". And second, you never quite know what quirky teenagery things (besides the full moon) will conspire to make various kids change the whole mood of the class/day/encounter.
I guess I have to give myself a break and know I'm doing the best I could be doing at the moment, and just the fact that I reflect and change things up is positive. But that's an analytic knowledge ... I'm sure I'll still be feeling that low-grade dread/worry/knot almost every morning.