Disclaimer: don't own
Anyway, here's Sirius' letter to Lily:
Dear Lily
Considering you were there when the charm was put on, I'm positive you know it wasn't me that betrayed you and James to Voldemort (and how many of his followers call him Voldemort anyway?). I just wanted to remind you of that fact. You saved Harry, I made sure to give Hagrid my motorcycle so he could get away safely (not that I would need it anyway).
Peter Pettigrew will pay for what he's done, even if I have no clue where he is or if he's even still alive. It's more shocking that I am still alive after being here with the Dementors, but I refuse to die until there is proof that the rat is dead and Harry is safe, wherever he may be out (which I hope is with Mooney).
I'm sorry for not just being the secret-keeper myself, maybe you could have lived. The only good thing out of this situation (and believe me, I almost could kill myself by saying this) is that we found out who the traitor is, despite the fact that it's of no use to us anymore other than the fact he can't be trusted again.
Padfoot - Sirius Black
When Sirius is in Azkaban, I'm sure a thought he thought was that James and Lily may have thought he was still the traitor despite being there when Pettigrew was made secret-keeper. He's going to feel guilty about a lot of things -but I'm sure he felt a lot of these thoughts. I don't think I've done a Sirius Black one, but I had. Running out of ideas and I could use some suggestions for inspiration!