Thank you for writing. You are describing what may be the symptoms of a clinical depression. It is not an unusual reaction to diagnosis of a life-threatening illness. Many cancer patients report feelings much like yours. I deeply sympathize. The fear and uncertainty of a cancer diagnosis and the pain of treatment sends many people into a very dark place.
You survived but you aren’t fully alive. You are already acting as if you died, when you have been given a chance to make the most of living. Rather than searching anew for meaning, your depressed self has embraced a philosophy of meaningless. The cancer didn’t kill you but your emotional reaction is stealing your life.
The way out is to get mental health treatment to help you reexamine your thinking and rebuild your life. Please look for a therapist who specializes in working with cancer survivors. In addition, check with your local hospital to see if there is a cancer patient support group. You are not alone. There are many others on the same journey who have worked themselves out of the darkness and who can provide you with practical help and emotional support.
Since you are a philosophical person, I do suggest you read or re-read Viktor Frankl’s book Man’s Search for Meaning. It may give you some helpful perspective.
I wish you well.