Background: I tried to do a self-control exercise that involved doing something boring like folding clothes while the exercise part involved repeatedly saying "focus" in my head, to stop daydreaming. However, after 15 minutes, my internal voice suddenly became weaker and weaker until it stopped to echo in my head. I tried to scream "focus" inside my head and it felt like moving a hand while in sleep paralysis, nothing echoed, while my front head (not forehead) felt weird. I am confused as knowing nothing about what actually happened and why.
Off-Question Notes: I have ADD, that sometimes something feels so boring I can't focus on it (The purpose of the exercise), but it doesn't seem to be the case, because I couldn't say anything else in my head nor think.
What is the cause of the phenomena whereby repeating words inside your head begin to fade from consciousness?