Hi there - it's an awfully sad situation & hardly surprising you want to cry all the time. I can't think of anyone who wouldn't feel as you do & it's certainly not selfish & you mustn't feel guilty about it. I think part of what you are experiencing is called anticipatory grief & it's very common for family members of those with a terminal diagnosis to go through. It's as tho' a part of you feels you have already lost her which in a way you have. You must just allow yourself to feel how you feel & of course spend as much time with her as you can. Tell her how much you love her & how much you will miss her - it will help you later. Let her talk about how she feels if she wants to - don't be afraid to talk & to listen. 

I hope this helps you at least a little bit. Carry on posting if you'd like to - people here understand what you're going thro' & will help & support you when they can. Take good care of yourself too. x