The last relationship I was in was with a woman who was very emotionally sensitive and demanding. She had frequent panic-attacks, periods of depression, and bouts of anger. For a while I felt like her knight in shining armor, there to protect her, but over time I realized that she had some serious growing up to do and really had little to no empathy toward me. I discovered that a relationship like that was one that I didn't want to, or need to be in, I got sick of it, left, and never looked back.

A healthy relationship is one of love and understanding, it sounds like, much like my past relationship, the one you're in doesn't have a lot of love and understanding. Cancer is scary, but illness and death are a part of life, and no excuse to treat someone you supposedly love poorly. Any reasonable person in your situation would likely leave him behind.