A struggle with sadness

So, at the beginning of the summer, we found out that my Grammie has cancer.  She went in to have fluid drained off of her lungs and they found cancer cells in the fluid.  At first they couldn't confirm what kind of cancer it was, but yesterday we found out that it is lung cancer and now it has spread to both of her lungs.  She was going through chemo treatments, but because of other health issues, she has had to stop for now.  She's been sick for a long time, but now it's so much worse.  She had breast cancer back when I was a little girl, and now she's just suffering so much. My mom is retiring early so that she can help out as much as possible.  I'm torn between wanting her to get better and wanting her to just not be hurting so much anymore.  I feel selfish when I think that I don't ever want her to die because I know she's been miserable for a long time.  She has lost tons of weight...almost 50 pounds in 2 months.  She needs to have surgery for another problem, but keeps getting it put off because she isn't strong enough for it.  I always thought that I'd lose it when my dad died.  But, surprisingly, I wasn't as devastated as I thought I would be.  I'm beginning to wonder if maybe I'll lose it when Grammie dies.  She's been such a big part of my life and such a good friend to me my whole life, I don't want to lose her.  I don't know exactly what to do to help her now either.  I don't understand why some people have to go through so much pain and heartbreak and others don't have as many issues.  I guess someday I will understand.