/lit/ - Literature

How do you make despair inducing dialogue? For example; I have a character shackled to an abandoned prison cell and is slowly going insane. There's nobody else around and her only hope of escape is the shackles turning to brittle rust in a hundred years or so.
So far I've got wildly (but still fairly realistic) rotations of emotions. Laughing one minute, screaming at somebody who isn't even there, crying the next. It's all just coming across a little cringe.
inb4 "post it", no but if it helps I'm getting a VA to act it.