More Than Just Muscle

Lets face it, more often than not the struggles we go thru in hitting goals in strength or appearance are very often mental battles. The human body is an amazing piece of machinery and its always surprising just how much we as humans can do with our bodies which brings us back to the mental struggle! Those paces around the gym psyching yourself up, the staring in the mirror or how about the self talks in your head,all of which are absolutes.
But what is it that separates the thought process of a monster and the middle age guy who is pushing just as hard as you? Where does that confidence or boost in mental strength come from? Is it all muscle or is it physiological in that the body and mind are connected in that way in which if you can make the brain believe it then the body follows?
Just for conversation sake, if indeed this is a mental struggle then is there some sort of mind hack or training for the brain in which you can push the body beyond its expected limits?
What do you guys think?