All reports suggest Sanchez worked very hard at improving watching miles of film and tried to be a leader in the off season. Remember his Jets west camp ? But he regressed on the field over the last two years which is the bottom line.
As to why he got worse, I'd say the Jets didn't provide as much support as they should have. Over the years the team had constantly changing (mostly bad) receivers, a declining OL & running game, Schottenheimer's poor play calling and a head coach who thought the offense was an afterthought. All that didn't help. But as a 4th year player it's on Sanchez to step up and he just didn't. End of story.
Maybe he just wasn't that good to begin with and the great OL, running game and Defense in 2009 / 2010 just covered up for him ?
In any case his confidence is shot a la David Carr. Sanchez will be nothing but a backup from now on and likely out of the league in a few years. Good thing he made huge bank when he did. His career is essentially over.