Hi Scott,
Wow man that's so sad... I'm really sorry to hear that. I fucking LOVE my cats, and get instantly attached to other cats too. Sorry it has to happen to you.
Here are too many pictures of my babies
We adopted both of these as kittens, about 2 years after we had to put our last cat down, who was 17 and had cancer. We promised ourselves we would never have cats again because it was just too hard, but after a year or so, we finally came around.
When the sad day comes, take her to your vet, and have her put down properly by a technician. They will give her a drug to calm her down, then one to sleep, and then a final one. It's peaceful. It's extremely difficult to do, but its the best thing you can do for your little friend.
Good luck.