He sits there, paralyzed, feeling, hearing the guilt and regret eat away at him. Unable to mov, he wants to scratch himself, claw himself open, make himself bleed, let the guilt and regret come out of him with his blood. He wants to spend hours seeing his blood come pouring out. He tries to close his ears against the voices telling him what he's done wrong, what he should have done, how he fucked up and what others must think of him now, how they must laugh at him.
He wants to move, and tries to, but can't, barely managing to squirm uncomfortably. He stops trying. He tries to lift his arms, pull up his sleeve to attack his arm with his nails, but can't. Something in his mind won't let him, and so he is trapped, frozen in his chair, forced to listen to whatever it is in his mind, wanting nothing more than to strip off his clothing and take a machete to this body he is stuck in.
Someday, he known, he won't be able to stop himself. Won't want to stop himself. Someday, he will attack himself, somehow, and let all his anger, his guilt and regret, all his mistakes, kill him. When he will kill himself. Until then, however, he must stay, controlled by his own mind, and he will obey it, for he has no other choice.