Saaaad!
I know I still have two cats that have been with me since I was extremely young, and it's gonna suck when they pass away.
All of our cats have the FTP virus, so at anytime they can get sick, and there's nothing we can do for them, my mother's even in a support group over it.
I try not to get attached to our cats, especially not anymore, it's just easier when they get sick and can't get better.