Engaged & afraid

I've been with someone who I love for years. The problem is is that I feel we are not right for each other, I never really have. The thing is we've been through so much together. She has a disease and was diagnosed not too long after we started dating, and I've been through it all with her, helping her for years now. I've given so much,time, changing plans, jobs, where to live... Our relationship has progressed as the disease has cycled from worse to better, but our vast differences have made the relationship hard, yet I've always been there for her. I know we are not right for each other, yet here we are, engaged to soon be married, because I felt, becuase I feel, like it is the right thing to do.. but not because I thought we were right for each other. I feel like it's the right thing to do but it doesnt FEEL right, but then again leaving her wouldnt feel right either, I dont think I could live w/ myself. This disease could get bad anytime in the near future... really bad. And the odds and history of it is that it will, that it is likely. It's made me depressed, I love her just as strong as I do for any in my own immediate family. How can you leave someone to have to deal with a heavy disease that you have such love for? But at the same time I feel like I'm sacrificing a large and important part of my life. Then I think that what happens if the odds are beat, the disease is beat. In this situation it would take years to know that, and I would have wasted my chance at love, at a family. And then I make myself sick bc I realize that's the same thing as thinking it to be a bad thing for someone I love beating the odds, to beat the disease, and I hate myself for it. I hate the situation, I'm depressed and lost.