Because innocent or not, Azkaban changes a man. And Sirius has a lot of bad memories to relive.
He's raw. Burned. Flayed open. Because every bit of pain hurts just a little bit more. Every bit of rage burns just a little bit brighter. Every ache of loss throbs just a little bit more.
Sirius can't remember what it feels like to sleep through the night. Sirius can't remember what it feels like to sleep more than an hour, what it feels like to sleep soundly. He used to be the one that slept through all the storms, the one that slept in until noon on Saturdays. What he wouldn't give to sleep even until six. Just once.
The nightmares won't let him go. Sometimes even when he's awake, they flash cruelly in front of his eyes: James, Lily, Regulus. Memories of his childhood: pain, anger, regret. He struggles to remember the happy memories. He has the faintest, blurriest glimpse of laughing with his friends, but beyond that… Nothing.
He's really sick of being angry. Ironically, he's so angry that he's angry all the time. He hates it, but it's as though his emotions are beyond his control. He's always worn his heart on his sleeve, but he used to be able to control the things he really really didn't want anyone to see. Not anymore. Now he simmers, perpetually close to an explosion. The smallest things set him off, and he hates that. But it's as though Azkaban took away his protective layers and left him bare to the elements. It took away his shields and left him raw and ragged. Everything affects him so much more.
He can't forget. Every little thing, every little sadness, no matter how insignificant it seemed at the time, throbs constantly. James. Lily. Gideon. Fabian. Marlene. Dorcas. Frank. Alice. Everyone he's ever lost. Even thinking about Peter hurts, but that's more anger than anything else. The pain makes him angry, and the anger is painful. It's inescapable, this vicious cycle that he's living.
He's so tired. So tired of everything. Tired of living in a maelstrom of emotion, tired of the looks of pity, tired of being cooped up.
He aches. All the time, he aches. He can't remember what it is to not hurt everywhere. He honestly thinks that if all the pain went away, he wouldn't know himself anymore.
The scars haunt him. Inside and out, they coat every inch. He can't escape them. They taunt him, reminding him of every injury, every injustice, every loss.
Sirius is broken. He's broken, and all the help in the world can't fix him, because this is what Azkaban does. It takes a man, pulls him apart piece by piece, and puts him back together all wrong. First it shreds, then it steals. It takes away every happy bit of a man, and it won't give them back. Then it sews the man back together, but without the happy bits, he doesn't fit back together properly. And he won't. Not ever again. Because even when he's escaped its clutches, it still won't give the happy bits back.
And Sirius is raw. Burned. Flayed open. Because every bit of pain hurts just a little bit more. Every bit of rage burns just a little bit brighter. Every ache of loss throbs just a little bit more. And he'll never be the same.