Even the best teachers fail. Talk to these teachers, and no matter how well the lesson went, they always can think of things that didn't go as well as they would have liked, things that they will do differently next time. But things get much, much worse when we collect the students' notebooks and look at what they thought we said. That's why Doug Lemov (2010) says that, for teachers, no amount of success is enough. They only teachers who think they are successful are those who have low expectations of their students. They are the sort of teachers who say, "What can you expect from these kids?" The answer is, of course, a lot more than the students are achieving with those teachers. The best teachers fail all the time because they have such high aspirations for what their students can achieve (generally much higher than the students themselves have).
I'm not sharing this because I think I'm a "best teacher". I'm sharing this because of the 2nd sentence. No matter how well things have gone, there are always things I can improve, things that could have gone better. This is one of the shared qualities that I see among my twitter/blog-o-sphere friends. The math blog-o-sphere is full of rock stars, but if you were to ask them, most of those teachers would never describe themselves that way.
This might just be what I needed to read right now... :)