When you can see it coming as a logical conclusion, everything else loses importance. The things you did to occupy your time and give you purpose you now know have no purpose. You just sit there frozen. Every time you have a notion to do something you immediately think "why bother?"
You wonder what will happen to your things and then you smile realizing you won't care, even though you would like some people to have certain things, you realize it would be worse for them if the did. You remember your father's things in your possession and they only occupied dusty corners in the basement.
You ponder what happens if you can't go through with it, but then you realize not doing so will only prolong the inevitable and make the extended time more unpleasant. You think you better guarantee you can follow through or the suffering in failure will not only cost you, but everyone around you too much.
You just want to go into oblivion and be quietly forgotten, your family better off for having resources that might otherwise be wasted on your sustenance. They don't speak to you anyway, you embarrassed them enough with your inability to keep jobs and live without their help. It would probably be a good thing if they simply stopped thinking about you anymore, like turning off a light in a closet that will never again be opened. The years of ignoring their birthdays and standing quietly by while they entertained themselves will pay a dividend of mitigation for their grief.