@Hogwild my dad essentially went downhill for 20 years with alcoholism. The last 10 years or so was due to a stroke from the ringer he put his body through with the alcohol and prescription drugs. I, too, have many many regrets about not doing more, not visiting more, not calling more, not looking him square in the face and asking him how he's doing. Truth is I was afraid to know the answer. The regrets kept me up and wore me down to the point where I would cry until I had no tears left in the first few weeks. Time is the great healer. It doesn't make the loss easier, but it does help you find contentment with your past choices and this new normal.
I'm fortunate that the last real conversation I had with my father was after we started exploring hospice care. I looked him square in the eyes and told him that I would love him always, no matter what happens. About five days later he was gone. Couldn't really even speak in the days after he deteriorated so quickly. In the times that I wouldn't be able to visit often, I was living far away from home pursuing a career that sustained me and made me happy. He was happy for me, even though I knew it made him sad that I couldn't be around as much. I take consolation in these good memories, knowing that I was doing something good for myself and that my pops was proud. It took me a while to come to this, mostly after speaking with my ma and asking if dad held it against me for moving away. She told me that dad would always say how proud he is of me. That's something I didn't know. It's normal to ask why we aren't/weren't better about doing things. It's normal to have regrets. That's the human condition, and no one is immune.
Your father knew he was loved and got to speak to his son on most days. I can't imagine much more a father could need or want in such condition.