Oh yes - a sad day - but also doing what I really feel is the right thing. About 7 years ago, my old Bert had a large cancerous tumor - inoperable and I could tell he was in alot of pain. He could not even go to his catbox, poor thing. So, I got my daughter from school, and together we took him to the vet. They shaved a small patch on his paw for the IV and I was allowed to hold and pet him as he left the world. It happens instantly with no suffering. I am so thankful that we were able to be with him and think that also helped him not to feel so afraid.
I know this will be a hard day for you! Can you put up a little memorial for her in your yard? We have a small cemetery in our yard and each cat has a "kitty" wooden cutout painted white with their names and dates on it and we also planted a pussy-willow there. Maybe you could have a doggie cut out? It is right under my bedroom window - behing where I sleep and that makes me feel better! Take care!!