I agree the lack of time at the 5 has hurt him this season.
He is too slow to guard anything else, trouble is though he is not a good small ball ball. Most fives abuse him.
1) he does not rebound
2) he can't rim protect
3) he does not have much length
4) production is poor for minutes played
5) useless at any shot save a post up on a guard or wide open three.
6) WE ARE A BETTER TEAM WHEN HE DOES NOT PLAY A LOT!!!!!!!!!!!!!!!!!!!
Last year he was on the floor against the Raptors in Game 7 because he could be trusted to make the smart play on offense and rotate on defense. This year it's very common to see Grant "try to do too much" and turn it over on offense (horrible passes, failing to catch good passes, being in the wrong spot and causing a teammate to turn it over, airballing floaters, etc.) or simply make bad defensive plays (missing rotations, taking what he thinks are "smart fouls" to stop transitions/layups while ignoring the amount of team fouls, etc.)
There was a few games ago where he made some deft passes I thought. But all the above problems still existed. A lot of times guys foul because they can't keep up.
Nothing is wrong with him, it is not his fault, this is who he is folks. If he playing large amount of minutes against NBA starters and good bench players he is going to be exposed.
As for being in his head, I don't think confidence can make a guy a better Athlete. Instead, he needs to embrace what he is and needs to be an effort player. He needs to play hard and smart every play if he is going to make it in this league. Instead of getting down when he air balls a layup or get swapped come right back at them harder. He needs to bang a bunch and make guys not want to challenge him and use his body to play tough and make contact anytime he can. Set bone jarring picks and play hard all the time. He also needs to quit whining so much to the refs.