This is why I believe that while training to fight should of course be injury free, it should also be painful.
If you experience enough pain on a day-to-day basis during your training, you become used to it and then pain becomes even less than a distraction to you; a dull touch that you barely feel.
In this way there is also no need to rely on your adrenaline to mask the pain.
Instead you accept it the same way you accept the discomfort of a doctor's needle; it may be unpleasant, but it does not truly harm you.
With this mentality, once a fight starts and you let your opponent attack you without striking back, and all the familiar pains of combat set in -the burning muscles and bruised skin and deep contusions and sharp aches from vicious blows- and you show your opponent nothing, not even a hint of any discomfort despite him giving it his all, you break his will to fight before you even strike him once.
Pain passes.
But every victory, once won, is forever yours.