Indeed, part of us DOES die when we lose someone we love, and being a different species does not diminish that feeling of loss, I know from personal experience.
Kevin, it really IS a PROFOUND experience isn't it? I mean.........the pain? It HURTS. But......that is the price we pay when we love someone, one of the partners in that loving relationship dies first, and pets (typically) die first in that case.
And Kevin? About feeling guilty for maybe feeling that you should have let him go earlier? DON'T. He wasn't THAT miserable, and you made the best choices that you could at that time.
Might I recommend a place that helped me when my beloved Rottweiler (Thor) was dying of cancer? http://www.indigo.org/rainbowbridge_ver2.html
And the main site is here:http://petloss.com/
This is a site with mostly women as participants, just letting you know, but they're VERY nice and helpful. I just thought I would suggest the place in case you're having trouble still.
I know that you were a good provider for your kitty, Kevin.