What you describe is not only normal but is an expected part of journey through a life-threatening disease. There are times of happiness and sadness. If the sadness is taking over your life and you cannot function and take any joy in anything in your day, then it's time to talk to your doctor.
It is extremely helpful to find someone that you can talk to about all possible outcomes, but I too found that most did not want to hear any "death" talk from me, only hope, joy, good thoughts, and unicorns It's very isolating to be alone with these thoughts and unable to express them to anyone. But people are not always in the same place we are as patients.
I eventually talked to a counselor so that I could actually say out loud "if I die" or "I don't want to die" or all those things that I thought that others in my life didn't want me to say to them. It was so very helpful for me to have a safe, nonjudgmental, place to express those feelings.
So, in a nutshell, you are not crazy, well at least not crazier than the rest of us