in my opinion if a player fumbles once he should be LESS likely to fumble again, regardless of his confidence. The majority of the time in football the reason a player fumbles is because he got careless and failed to tuck the ball away properly. Once a player fumbles he is going to be extra sure he tucks it away again so it doesn't happen again, which is going to decrease the chance that he fumbles. How confident you are really should play too much of a role there.
Now, interceptions is a completely different story, and confidence should play a huge role there.
Now, interceptions is a completely different story, and confidence should play a huge role there.