Did any of you resolves differences with those in your care? When underneath a sick physical body sometimes is the same arrogant, opinionated, judgmental person.
But living with the potential regret of not resolving things would kill me.
I was not abused only that he has always been an absent father that seemed to only care for himself.
There are differences that cannot possibly be 'resolved' in one lifetime with an abusive parent. What you may try to work on is forgiveness to some degree so that after he dies, YOU do not harbor guilt and regrets. But to say that it's 'killing you' NOW is to say that your life is already being ruined, today, by your father and that means forgiveness isn't likely.
See a therapist who may be able to coach you through a process whereby you learn to take care of, and love, YOURSELF now. Devoting an entire lifetime to someone who can't love you back begs the question, why? What about ME? The therapist may be able to help you sort all that out now, while dad is still alive so you can find some forgiveness for yourself AND for him.
Good luck.
If you are going to care for this person, you may have to go "grey rock".
It didn't change my dad but, it changed me to forgive him and help him when he needed it most. I wouldn't be able to live with myself if I walked away when he was in need.
You have to decide for yourself how your relationship will move forward. I had lots of boundaries and that was hard for my dad but, I couldn't trust him to not take advantage if I didn't have them in place and enforce them.
I do recommend having a really good idea of what you are and aren't willing to deal with, learn to say no lovingly, without guilt. Remember that you are an adult that is no longer under parental authority. It needs to be an adult relationship and not a parent/child relationship.