Sirius spent much of his time in prison composing letters in his head. It was a good way to pass the time and was somewhat therapeutic. After all, since he could never send these letters he could say whatever he wanted to.

Remus,
I'm sorry I believed it was you. And I can't say I blame you for believing it was me. I miss you. I want to get out of here so badly. I want to sit with you and drink tea and talk about quidditch. I wish we could have raised Harry together. What happened to him? Is he ok? Are you visiting him, Moony? Is he growing up healthy and strong? Are you taking care of yourself? I hope you are and I wish I was there for you.

Dear Harry,
If I were able to send this letter to you, would you know who I am? I hope you don't because I don't want you to believe the lies. I remember holding you, minutes after you were born.

James and Lily,
I failed you! It wrenches my heart every day to know that I failed you, that I convinced you to pick Peter.