My English is not good so I do the best I can. If you find any errors, please tell me in the comments. Thank you.
I want you to know that you should forgive yourself. You should understand that it wasn't your fault. We all thought it was the best plan, we totally trusted Pettigrew, but he ended up being another follower of the Dark Lord.
Sirius, you are a special person with special feelings sentenced to 13 years of misery for something you didn't do. Forgive yourself because we never blame you. Although we would have preferred you not to leave Harry alone, I understand why you let yourself be locked up in Azkaban. Your best friend and his wife killed by a traitor who was just another of your friends. I'm sure I would have collapsed too.
Regarding Harry, thank you for taking care of him and protecting him during the time you were among the living. For giving him that family love he needed to be a little happier, even though it was later taken from him at the hands of Bellatrix.
We love you Sirius, you are James's best friend and I have to accept that over the years you have become more bearable. Don't blame yourself for everything that happened. For our death, for having left Remus and Harry alone, it is something that was not in your hands, but you did everything possible to give my son something better.
I thank you, Sirius, for the time you invested in your godson. For giving him smiles, hugs and affection. For showing him that there are people who care about him beyond his fame and money. I love you, Lily.
*Happy birthday Sirius Black