Oh no, I'm so sorry. You have me tearing up.
it's a crappy answer but it takes as long as it takes. I just lost my mom and a customer told me just this morning "The hurting will go away, but the void will never be filled". I think it's the same for any creature who stole a part of our hearts.
Your grief is that much worse because of the reasons surrounding it, but I know you made the right decision. The fact that you're so broken up over losing your beautiful boy tells me you did not do this for selfish reasons. Safety is not a selfish reason. The fact that he had some chronic health problems is also not a selfish reason.
I believe in quality vs quantity of life.
In a perfect world, we'd have both, but if we have to make that choice, we have to choose to give our friends a good life - even if that means it's not a long one.
It's ok to feel sad, feel cheated, feel guilty. This is all normal. But know in your heart you made the right choice. He did not suffer, and he loves you still.
Look forward to your new pup! I hope the breeder helps you choose just the right puppy. You are not replacing Luther, nobody will replace him. You're just giving your heart another creature to love.