poor you, YG. It's very easy as a teacher to focus on the criticism or the bad things that happen in one day, isn't it? i've really tried to stop doing this - easier said than done, i know - but it was really starting to get to me.
Try to focus on a good thing that happened in the day as well - i bet there are far more than one - what about the child who did get the part and how happy they will be, and all the other kids who are just happy taking part in a show like this because they are going to love it?! Forget about this horrible thing and think about all the other parents who will be full of admiration for the effort you and your colleagues are putting in to make their child happy and confident.
I was in the CRAPPEST mood when i got into school yesterday and felt like crying. then i had my first lesson and the kids were so nice and were trying so hard (they were performing poems as raps and they really made me laugh) that all the crap feelings melted away. It can be the worst job in the world at time, but remember it's also one of the best on good days