Every teacher knows that teaching is an emotional roller coaster -- we are thrilled if a class goes well, depressed if it doesn't. But the reality, I'm afraid, is that every class we teach, even the good ones, is a failure in one respect or another: some students found it too difficult or too easy; some students were engaged, but didn't learn what we had hoped; some students picked up some skills, but not the underlying concepts; and so on. We learn a lot by paying attention to these daily challenges -- in fact, they are the main avenue to professional growth.
On the other hand, obsessing about them, and trying to anticipate every possible bump in the road is asking too much of ourselves. Becoming a better teacher, paradoxically, is only possible if one accepts that each lesson we teach will of necessity be imperfect. We learn a lot more by keeping the job manageable, and staying in the profession for many years, than by making unrealistic demands on ourselves and burning out. As they say in French, the best can be the enemy of the good. Teaching is not a good profession for perfectionists.