Sirius has a lot of time to think about the mistakes he has made in his life while in Azkaban. They are numerous and haunting to think about. He can pinpoint so many things he would have done differently, from simply keeping his mouth shut and avoiding a beating as a child to having better control of his temper when it mattered as a teenager. Sometimes he wonders if it would have changed things in unpredictable ways, made them worse but he can't imagine how. Maybe if Remus had caught up with Snape that night in the tunnel their lives would have been changed but he has no way of knowing how it would have impacted Peter's friendship with James. Would he still have been trusted enough to keep their secret only to give it away upon his knees?
Lily and James might still have died and Remus would have probably have gone to his grave long before for what he might have done, Snape along with them. They say that Snape is a spy, or at least he was, so that might have changed many things but Sirius has always been narrow-minded and cruel in his own ways. He doesn't want to think of Snape as important enough to change so much, not even when Snape is the free man and Sirius the prisoner. He still things himself superior when really Snape has done better for himself and that would make Sirius' blood boil if he wasn't frozen in his cell. More importantly Harry still might be an orphan and that is what really matters to Sirius. He can take responsibility for his mistakes but Harry is innocent and doesn't deserve to be the victim of his godfather's stupidity.
For someone who is so intelligent Sirius has a terrible habit of acting before he thinks. When he was younger it just got him grounded, lectured and sometimes cursed by his mother. Now it has him in prison and Merlin only knows where Harry is now because Sirius couldn't stop, think, and do what was best for him. He should have stayed, fought for him, argued with Dumbledore or just outright kidnapped Harry. The law was on his side but he had thrown it all away. Money was never an issue for Sirius. They could have disappeared and tried to be happy somewhere. Time is hard to keep track of among the screams and the cold emptiness but Sirius reckons Harry must be four or five by now. Muggle children go to school at that age and Dumbledore had sent him to his aunt. He wonders if Harry likes it, if he gets on with his cousin and what sort of friends he has. Sirius would know these things if he'd stayed and fought for him.
Sometimes he doesn't think at all because it is too hard. The Dementors take his happiness, his thoughts, everything that makes him Sirius and all he can do is transform and curl up and hope it ends soon. He lies whimpering with his paws over his face, scratching at his cheeks and scuffing his back paws across the stone floor. It is easier to be a dog but easy is a heavy word in Azkaban. He's spent days lying motionless in the corner thinking only a few hours have passed. When he feels better and returns to his human form he reminds himself he is innocent but the voices in his mind whisper that no he isn't, not really. He was the architect of his own downfall. He just isn't guilty of what they say he is but that certainly doesn't make him innocent. He allowed himself to be put in the situation that landed him on the island and he can't claim to be the victim in everything when he thinks about that. Some Auror he was. He was meant to be above grief, infallible, capable of rising above mere emotions to get the job done.
His thoughts drift when he is able to think, innocent and guilty, and the faces of the people he put in Azkaban float across his vision shaking their heads and then cackling. How many of them had been in the wrong places at the wrong time and looked guilty enough to be thrown away without anyone making sure they were the right person first? Perhaps karma did exist and this is his punishment for the pleas he ignored and conviction he was sure justified his actions during the war. He is hardly the first to be sitting in a cell without a trial behind them. Panic and anger take over during wars and it is about getting one Dark Witch or Wizard off the streets before another can take their place. It was a numbers game, simple as that. Monday mornings were death tolls and recruitment figures and the more Sirius thinks about it all the less angry he feels about his situation. He can only blame himself when it comes down to it. Now, later, he can see that he should have gone straight to Moody with what he knew. Moody understands decoys and placing the heat on someone that can stand it, knowing they won't crack and even if they did they will buy even more time by not knowing anything in the first place. He might have reasoned with Dumbledore.
People were meant to think it was Sirius protecting the Potters but it is naive of Sirius to think he couldn't have persuaded them it wasn't. Mistake after mistake and all the time in the world to rot and think about them. That is what he's left with. The legacy of such a promising child and talented teenager is a broken young man who can't tell one week from the next. He wonders if it gives his mother satisfaction now, to think he turned to Voldemort like his brother did. Both her sons are gone now and she is left with only Narcissa as family now. Perhaps she regrets things too. Maybe she feels she made mistakes. It doesn't matter. Nothing does anymore. He tries to remember he is innocent but he knows he is only innocent of the crimes he has been imprisoned for. The innocence just reminds him of who he is but it is the guilt that reminds him of what he is and keeps him pinned to the corner of his cell remembering his every action over and over again. A baby's laughter at a pitiful birthday party and Peter's cries in the street. He is still sane compared to the others but he thinks it might be easier to go mad. The mad ones scream night and day but not about anything that makes sense. New demons chase them in their dreams. They don't have to remember their mistakes. They don't have to live with them.