I don't suit to be one...

Tonight , at first just like other trainings, I start the exercising of the class in Taekwondo.
Brown 2 and above were taken away to train on their optional patterns.
Leaving just white to blue-red belts...
After exercise, me and Li Yun look at their pattern and pattern kick.
Heard from her they were quite...bad.
Then I decided to take my group to the back and train their pattern and pattern kick.
The white and white-yellow were quite playful though...Tried my best to stop them to play.
Who knows that tonight was mock grading thus I felt a bit of stress but I can handle.
In the end, when my group went for the mock grading, none passed.
When Suriana said: 'This whole bunch of people go there and cross.', I felt so heavy hearted...
I felt so useless...Li Yun and Suriana taught them so well them their group passed.
After class...I tried to hold back my tears.
Finally when I got into the toilet to change my clothes, I cried.
As I don't want them to know I cried, I pretended to be happy and said: 'I'm going first =)'
But actually I felt so hurt...So useless...
Why is it that what I do results in failure?? I'm so useless...
I shouldn't even be a Mdm at first...I taught them wrong all the way...
I know how to do white belt pattern yet I taught the white belt wrong...
Maybe I should stop being an instructor in Taekwondo...
When I heard her say that...I felt like bursting in tears...='(