And normally, I am quite good at Oral and Written English as a non-native speaker when I am not too tired.

You do very well for a foreign speaker of English. I will congratulate you on that. However, I now realise why. I'm afraid, when the training centre or university says to you 'your results are most unreliable and the students are complaining' you will not get away with saying, I get very tired, and after all I am not a proper English speaker.

As for the rest, it's best I ignore it. Please leave the English teaching to native-born teachers.