My father passed away from colon cancer more than a year ago. His cancer diagnosis, which came 4-5 months earlier was especially hard to take. In his last 2 months, he was admitted to complex terminal care in hospital. Although I visited him often (2-3 times a week), I still feel I should have visited him more. Taking off work however, was not an option. For my late mother, who passed away nearly 5 years ago, she too was admitted to hospital and then transferred to a nursing home for her final 2 months. I visited her even less...partly because I didn't think she would pass away so soon after her diagnosis. For both my parents, I feel guilt over this and wonder how others deal with this guilt after their loved one has passed.
Use what you have learned to move forward, not to beat up a past self who was only doing what seemed reasonable at the time. Spend your energy being a caring friend and loving relative to the people in your life now. Don't waste it in guilt about what you cannot change.