Dan, there isn't one of us who wouldn't wave a magic wand over you if we could. Not one of us, however, is a professional, so anything we say to you is worth what you pay for it. Except this: Get thee to a trained counselor who knows what to say, how you feel, and what to do about it.
No question you are in one tough spot, Dan, and our hearts collectively ache for you. Please ask for help so this part of your life will not be spent being depressed. The cancer most probably cannot be cured, but your depression can be lessened for sure and certain. I do not often speak for anyone but myself, but in this instance, know that we are ALL pulling for you.
Joanna