Analysing failure.

What a hideous week. I had no less than 3 failures yesterday. Of the three that failed, one perhaps I could understand. The Of her two gave me a complete body blow. How can I get it so wrong??? We practiced well and I genuinely thought we were in for a merit. I feel like I know  nothing. I feel like the worst teacher ever... a charlatan.... How could I get it so wrong???,,_