Sasha, you could not save him. You could not be his "wife support."
More likely, your DD would have watched up front and personal his falling apart, verbal and possible physical abuse of you, and who knows what else. You protected her from that.
Go back to the basics friend. You did not Cause it, you could not Control it, and you sure could not Cure it.
The idea that one should live a miserable life and sacrifice their own mental health to try to keep someone from going down the rabbit hole is just not sustainable. Your DD has been given a wonderful mom who has protected her at all costs. You have minimized the amount of damage she was exposed to. You did more than anyone I have known.
Stop blaming yourself. Because someone has passed does not negate the things they did while alive. That sounds very harsh, but it's so true.
Sending you very big hugs.