This has made me take a deep breath. So sorry for your loss. My Dad passed away over eighteen months ago now. I will say that it does get easier with time, hard to put into words really but it will in due course for you. My Dad was the same age as your Father and having read your post I'd say there were some similarities, mine also had lung cancer which spread to the brain. I remember when he was in hospital and was on steroids he did perk up better than we expected but it was only a short term solution, since they can be toxic.
As someone else said, be strong for your family and I sincerely wish you all the best for now and beyond. I do know how you feel.