Lilbear I can see and feel in your words the desperation and struggle with all you are doing and the pressure that rains upon you daily. My words and thoughts pale against what you are feeling, facing, and doing. I have been a caregiver for my wife and toward the end I too felt the sense of no life but I knew that my life for the time Donna had left was dedicated to her care. At the time it didn't help but after I took some pride in knowing that I was there and did what I did. That said you have both parents who are in need and you have to face what they do and say that rips the fabric of what a child is to a parent.
I found a counselor at cancercare.org and for three years she allowed me to share and weep and begin my grieving well before Donna died and after. I cannot recommend the professionals who work with cancer patients and caregivers more highly. They understand what we face and how to manage it. They give us the vision to see between the lines or our daily struggle and those issue which lie deep within our hearts. And I may have resources at hand to help you beyond listening.
Please keep sharing with us if for nothing else to exercise your voice and heart with those who have gone before you.