I don't think it's mental. I think his form is broken. Not bad, but slightly.
His right elbow is slightly too far out. It will naturally be out some, but too much is a problem, and more importantly (like a golf swing) it must be straight when you're coming through. Some guys can adjust it from the cocking part to the follow through, but he obviously can't. That means that that his right forearm is not in line with the shot.
Maybe more importantly it means he puts his left hand on the front of the ball to balance it. That's a big no no. That left hand will always affect your accuracy because it creates side spin sometimes and a jerking motion that isn't smooth.
This is an extreme example of those things all exaggerated. Left hand on top of the ball. Right elbow flared way out, causing the right hand to not be in line with the goal (pointing left of the goal).
This is Briscoe. Notice how his right hand appears to be pointed left of the goal. See where those seams are pointing?
This is Steve Nash and the way it should be. Left hand NOT in front of the ball. Notice how, while his right elbow is flared out a little bit (again a little is normal) his right hand is perfectly in line and underneath the ball.
Some of it is mental. His form isn't bad enough to be shooting FTs this bad. He can do better. With practice and confidence, even with the flawed form, he could shoot 50-60%. But he won't get higher than that until he tweaks it a little bit.