When do you say that enough is enough?

My grandmother. She is a great woman. My early childhood years were spent only knowing her through yearly summer visits for a couple of weeks at a time. In which - she was always spoiling and getting us out of trouble with my mother. I didn't really get to know her - to have actual memories of her - until we moved into the house with her and my grandfather. And since then - I've come to love her more than I can imagine. She is the reason that I will go and stand in a Starbucks - just to smell the coffee - even though I hate the taste of it. She is the reason for my love for chocolate pop tarts (she let us eat them for breakfast after my mother went to work), Vienna finger cookies, ring Bologna, and chicken and dumpling. Living with her is the reason that I still love to hear the clicking of an old round dial phone when calling someone. All my Missouri childhood memories are connected to her. Of her letting us (my brother, my two cousins, and I) have the freedom to wander the entire neighborhood - from her house, to my cousins house, to my house (when we got one). Of her making sure that we were home for lunch with my grandfather once he retired - to watch Andy Griffith reruns. Of her being the buffer blocking all the bad things from my parents divorce - so my brother and I wouldn't have to watch. For all these things - I have love her like another parent.
Within the past couple of years - her health has declined. She was a smoker until I entered high school (she quit when the doctor made my grandpa quit). And she has had COPD for a while now - which made her a little slower moving - but didn't diminish the woman I loved. Within the last year though - the doctors have found a growth. It's in her lungs. And everytime they check it every couple of months - it's grown. The doctors have said it's cancer (although my grandma won't say it) - but she is not strong enough to try any treatments that are out there. Not surgery, not radiation, not chemo. So - now we just wait - and watch. Last night she got pretty bad - couldn't catch her breath, even while sitting. So, she was taken to the hospital.
This has happened before - and I've known for a long time that she is not okay. But - I've never had to be party to seeing how bad she can get. I've never actually been there to witness it. Until last night. And it's breaking me.
I caught myself last night - saying my prayers before I fall asleep - praying that my Grandma would be okay. But - I stopped. The thing is - I can pray for her to get better this time - and for her to go home again. But - she'll never be okay again. I know this - but yet why did I pray for it? I know that if she gets better - she will just continue to have these attacks more and more frequently until she goes. And so that's what I wish upon her? I want her to have months or years of pain and fear - before she can finally give up? Why would I want that for her? Because - she'll never be "fine" again. It's because - I'm not ready for her to leave us. I can't pray to have her gone from me! Isn't that wrong? And I can't make myself do it! How do you know when to say, "it's enough - she can't do anymore"? I don't know. I don't know how to pray for what I want - and what's best for her. Just because I don't want her to leave - doesn't mean she should have to suffer.
How do you decide what to pray? How do you accept that someone is going - and stop fighting it? Because - my family brought me up to fight for things. So - how do I give up fighting for someone who means more to me than I do to myself?