It wasn't long ago that it was. It wasn't long ago that many (even on here) were regarding Hart as the best in the world while DDG still had something to prove. The turnaround from the two has been quite remarkable.
Best in the world was probably only from people who doesn't watch anything outside of England, but he likely was in top 5 keepers in 2011-2012. De Gea last season was probably even better than him in previous season though.
Anyway, going on topic, I think that it is a mentality thing and he is believing the shit media thinks about him. He will never be a Gordon Banks, but he could be a very good keeper. It will all be depended on him and in the manager of City. Probably dropping him for 5 games or so wouldn't be the worst idea. It worked with De Gea.