Why does death or dying want to die always pop in my head? i'm not suicidal

Anxiety and flight: You are fatigued and have cancer. It is natural to think of death. The underlying emotion is not depression, rather it is anxiety with a flight into death fear. It's been said that every anxiety is based on a paradox of a fear combining with a wish. You are clearly experiencing normal anxiety. Seek counseling with a psychologist or a chaplain and let your primary care doctor and oncologist know. Best wishes to you.