Friend keeps undermining my cancer? Its a serious issue and i don't understand why.?

First of all, I'm truly sorry that you've had to endure so much. A close friend of mine has cancer and I know how difficult it can be. I'm also sorry you have to deal with this person. I've got to be honest, I CANNOT stand people like that! Ugh! I think I'd just stop sharing anything with her (as you said). I wouldn't bother being friendly, either. Civil, yes, but I certainly wouldn't go out of my way to talk to her. Eventually she'll get the point.