That is so sad. Cats have been very important in my life, too. Our cat Puma is getting up there too--his health is OK, but anything could happen anyday.
If you can be there when she's put to sleep, please do. I was so depressed when our family cat Smokey was put to sleep that I didn't go. I'll always regret that. I never got to say goodbye. If there's a Heaven, I plan to apologize to her for being such an asshole.