Theres no way I could know 100% that he has confidence issues however this happening along with his general demenaour in my opinion leads me to believe he lacks confidence.
Then you really shouldn't be using that as an excuse for him for anything.
I could state his attitude sucks which you would dispute strongly, but the truth is that neither of us know the truth.
He might just be a shy boy for all we know, he might have just hated Koeman and also his confidence may have been shot to pieces playing in a losing side under Martinez, or from going to being "Golden Boy" to somebody who couldn't even get a look in for his country any more.
Let's just hope that he gets his head down, produces on the pitch and turns into the player he once looked like he could be. Preferably for Everton, but if not then so be it.