The boss said I was too stupid to use my brain or be flexible. I'm not wanted because I'm diligent and earnest. He said I can only use my hands, but I can't think.

Maybe you are such a person, after all, the boss must have this idea if he can say so. Everyone should constantly learn to change themselves. You should also refuel, listen carefully to suggestions and be good at thinking and analyzing things. Change your work habits interestingly, first understand what the boss has arranged for you to do, then form a good habit and use your head to analyze before you act. Reasoning is a good embodiment of habit and ability.