nah personally i would be honest. im not sure how long youve been dating her or how intense your feelings, but IF I LOVED HER, i'd say:
"well although most people are raised on the romeo/juliet story, the reality is that people are tough. In history, our ancestors have endured war and peace, famine and plague, and all manner of tragedy. Humanity... we have to survive, even in great tragedy. We have obligations, to our family, or friends, and most importantly, ourselves.
"So although i would be utterly heartbroken and i cant imagine how depressed, i would survive. I wouldnt kill myself like ROmeo, nor go insane. THe first months without you would be the hardest, but over time i would heal, and grow and survive.
"And eventually i would find a way to smile again and even be happy again. But it would be a different kind of happiness. My life would never be the same.
I cant imagine life ever again being as good as it is with you, althoug im sure it will be "good enough" to make life worth it, worth enduring the tragedies.
Like a shattered vase, i would go on, i would eventually be mended. But there would always be the scars - the glue-filled cracks - on the vase. Yeah, i would survive, but"
[reach over and put your hand on her cheek,]
"but i am SO glad that i wont have to go through that , and even thinking about this makes me so grateful at what i have."
[BOnus points if you shed tears at the thought of her dying and let her see.]
PS, did you answer her yet? how did it go?