Well I have my sympathy to OP, as I know how painful is dealing with ET especially out-station.

ET long delay is quite common problem I suppose. ADD-BKK flights when it gets delays it will be 7-8 hours delay. It is wise to always check if the flight even depart before you go to airport.

I was rejected for my return flight from Bangkok because they said I haven't paid for my tickets hence they cancel my ticket.

Enough dealing with ET. I will avoid them even the fare is damn cheap