Every once in a while I go through the "I'm obviously the worst teacher in the world" phase. And why do I say that? Because there are times my students don't practice; or complain about doing scales and technique; or tell me the pieces in the lesson book are "boring"' and just in general don't have the same commitment to piano as I do.
The only way for me to get out of those blue funks is to realize that I'm doing the very best that I can, and they're taking as much as they are able to use. Of course I do my best to change those behaviors - like practice contests, technique contests, supplementary music, etc., but those are the times that truly try my patience, and make me feel stoopid.
Also, I find that when I've reached that point, that I've been trying WAY too hard, and I need to relax a little, and work towards their assimilating new concepts, rather than learning new ones.
Of course, I'd like to see the learning curve ascending every week, not becoming a straight line, but the hardest thing for me to do is realize that some things are out of my control!!

....these are true feelings. Feel free to slap me out of it.