You know, you shouldn't feel bad for wishing her pain to go away. That's really what you are wishing.
I know it's probably sounds cliche, but perhaps some counseling would be in order for you. Where I live we have a place called "The Centre for Living with Dying." Look the the pdf files here:http://www.billwilsoncenter.org/thecentre/resources.shtml

I'm sure your school has a resource for you. Perhaps you can use them.