Is it possibility of failure or fear of the pain?
You might be able to run a great plan and address fear of the pain. Often, learning to calm yourself on the starting line leads to not being aggressive enough when it counts.
You always have enough adrenaline at the gun, despite trying to calm yourself, because it is a specific moment that comes, like it or not. Your body is good at fight or flight at a specific moment.
Trying to calm to start the race often leads to a wall at about 2 miles because you are telling yourself to put the adrenaline away and end up never using that fight or flight power.
Actually, this fight or flight adrenaline at the beginning of the race is why may HS runners go out too fast.