What causes words repeated in your head to start fading away from consciousness? 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.

Question

What is the cause of the phenomena whereby repeating words inside your head begin to fade from consciousness?
If you use a slightly different (and to my own experience more common) phrasing of this phenomenon to look for information you find immediate relevant results on Google. Rather than referring to words fading from consciousness I've usually heard it being stated as repeating a word often makes it lose its meaning.

The first hit is a Wikipedia article on semantic satiation.


  Semantic satiation (also semantic saturation) is a psychological
  phenomenon in which repetition causes a word or phrase to temporarily
  lose meaning for the listener, who then perceives the speech as
  repeated meaningless sounds.