Soldato
For sure, I wonder what the stats are, regarding the reasons people fail. As you say, at the very top of most fields it can come down to the psychological differences. Some players are never the same after a serious injury, some boxers don't know when to quit or never are the same after a big KO etc.
Yes, i think it can make a big mental impact on people. My wifes ex's brother was on the books as United around the same time as Neville, Beckham etc and got released at 19. He had offers from a lot of smaller clubs and yet his mental state at being released was ruined and he turned them all down and gave up on football, when he could've probably had a decent career.