I am glad you are filling your time with your own life now. It is important that you NOT help youngest until he starts to straighten out his life. His claims that you have not helped him are pure nonsense and if you ever start to doubt that, let me know. I will remind you of ALL you have done for him, and how far and above you have gone for him. He was given all the tools he needs to live a happy and successful life, and not using those tools is a CHOICE. Sure, addiction is a disease, but you can CHOOSE to heal and rise above or to stay sick and wallow. I hope that he stops wallowing someday, but that is not for anyone but him to choose.
Your grands are precious and being their gma and helping raise them and support them is another way you help difficult child, but it is a way he totally doesn't care about and that is on him. He will reap the problems he has created in his relationship with his children when they are older, and that will be smething else for him to deal with and you to not be blamed for or involved with.
I hope he figures out how to fix his life at some point, but even if he doesn't, you and husband need to go on and enjoy your lives. Your universe does NOT revolve around the things Youngest thinks you owe him. He has taken more than enough of your resources and thrown them back in your face time and time again. Someday he will hopefully see this and give you the apology he owes you.