Oh man. In July, my fathers passed due to pancreatic cancer. We had been estranged for years and years, at one point I believed him to be dead. A few months before his passing, I had been putting in effort to reconcile with him. He has two granddaughters that he never had the opportunity to meet, and he hadn't had the chance to see my two sons for 3 years. Anywho, I had tried to call him twice the day before he passed, but he never answered the phone. I remember being pretty upset at the moment, but in hindsight, he probably didn't have the ability to do anything.
So, if I had to redo any one thing, it would be to put the effort into reconciling with my sick father before he left this earth. I'm not saying I would have been successful, but the worst feeling in the world is failing and knowing that I should and could have tried harder.