You have had to deal with the fallout and loss. My mum died from cancer when I was about 25. It was a female only variety - I forget the exact name. She had an initial onslaught which was diagnosed in reasonable time, an operation that she described as "quite unpleasant" and a course of radiotherapy. I should probably point out that "quite unpleasant" is a euphemism.
... remission ... all clear ... ... oh sorry its back ... palliative care. Funeral.
Losing a parent early is horrible, even when you are nominally grown up. You should have at least 20-40 years as an adult with your parents, if only so they can ram the point home about what responsibility really looks like as you foist your ankle biters off on the oldies so you can get some respite.
Sorry, we were discussing the fall out from addiction. Yes, it is horrific. Your dad gave up and that is quite an important thing. That shows he tried to do his best for himself and especially - his family. He was old enough and ugly enough to know exactly what was happening ... and he did his best, his very best: that I am sure of.