Hi Hopenyc,
I went through a lot of similiar guilt feelngs when my abf's stepfather was diagnosed with cancer a few months ago. I was ready to bail out of the relationship then the cancer happened. I tried to do all the right things for him and his family. I really went in to a bit of a codie binge over it all. In the end I probably should not have bothered. He hardly even speaks to his stepdad now because the cancer has made him "grumpy and selfish". Yes, abf has labeled the cancer patient selfish.
My relationship with abf has ended anyway regardless of my efforts to the the "right thing" by him and his family during those early days.
If it is time for you to end it you will know I think. His life will continue to "happen" with or without you. His sobriety is his responsibility regardless of circumstances. You can't change that - just like the cancer can't change it either.
Sending you big HUGS - I know that guilt feeling all too well. :ghug3