You stayed in wizard prison for 12 years of self sulking about the past, then you get out and buy him a broom like that will fix anything, you're not able to take him in (okay that's on pettigrew but malfoy got away with sm and someone of the house of black can't what's the point of being wealthy then?) and be a parental figure for him that he desperately needs no you're trying to replace Harry for your own selfish reasons, trying to get your dead best friend back, you try desperately to be that cool uncle which is great in a teens eyes but god such a fucking loser, you barely tell the orphan anything about his parents or anything about the potters that you lived with (yes James parents but also nothing about lily) only about the "cool" pranks, you call your godchild boring for not going to hogsmeade while Voldemort and his crew is on the loose, you stay in your parents'home fighting with your mum's portrait all day and acting like a 12 year old on Order Meetings and then you get your godchild in danger and die?

Mind you, Sirius and his cousin are the only ones known to where ever able to break out of Askaban. So they both had a lot of magical power. Why wait 12 years when a boy needs you? You don't have time to self pity yourself, you can do that when you die.