"My lungs just weren't there today."
"I just couldn't push as hard as I wanted."
"I just wasn't tough enough."
People say this kind of stuff all the time to justify poor race performance and although I'm not trivializing the importance of attitude and mental strength in the sport, I would like to STRONGLY stress the mind and body are one in the same.
Some athletes are blessed with a dogged determination and will. They fight to the death. Performance for them is worth entering deep into the pain cave. The mind is an incredibly strong tool to overpower the body and these strong willed athletes push their bodies to forge on even when every cell screams, "Stop!"
When the body doesn't respond though they feel that they only faltered mentally, but their is indeed a physiological reason for whats happening: endocrine burnout.
Athletes with strong discipline have a tendency to push themselves hard in training. Sometimes they feel recovered physically but in reality they show up on race day fatigued. This is hormonal fatigue.
Runners get their ability to push hard from their hormones. These hormones are primarily epinephrine, nor-epinephrine, and testosterone, as well as cortisone and a myriad of others.
Read this excerpt to learn about the hormones:
These hormones are what make up the fight or flight response. They make your heart beat faster and allow you to release fuel like fat and glycogen into your blood stream. They give athletes the ability to dig deep and really enter the pain cave in a race. Well rested and trained athletes can push themselves at 100% when these hormones are readily available.
When athletes train too much and ignore the signs of over-reaching and / or over-training these hormones become released in smaller quantities.
The mind can't just "force" the body to perform optimally because in essence the athlete is experiencing a shortage of necessary fuel for your race. These hormones which runners usually use while racing are not there in similar quanity because the hormone system is fatigued and won't dump out the same level of hormones as it would in a non-fatigued state and therefore the runner can't dig as deeply as normal.
For more info I suggest researching flight or fight response and stress adaptation.
You can't push at 100% all the time. The body can't differentiate life stress and training stress. Allow balance and recovery.
Racing too frequently is a major cause of endocrine fatigue. The body needs downtime to restore hormone levels.
Good luck in your training!