I too can appreciate that writing this was difficult. Especially, since your dad was so affected by the death of your mother two years prior to his own. The death of your mom was a catastrophe for your dad and it seems he never really could deal with it and live.
My dad died 59 years ago this coming April 22nd. Loosing a parent is always hard. Your story is particularly compelling because it defied expectations and you in particular were hoping for Martin to get to know the father who raised you and that was not to be...
From the point of view of a son who is a nurse and had familiarity with taking care of patients, to being a family visitor must have increased your sense of futility. Your continued difficulty dealing with this topic is completely understandable. But you have taken another important step.
My only advice from nearly 59 years of experience is to remember the good times and to remember the times your father took pride in your accomplishments and his good wishes for the future. Death is part of life and living well and appreciating each day as a gift is how I keep on my journey.