I know exactly how you feel Chewy. If you haven't yet, read my dedication page for my Previous Siamese cat, Charly. He lived till he was 16-17.
http://www.tgeweb.com/dedication.htm
I couldn't put him to sleep. I just could not do that. I held him and was with him until his last breath. That was the worst moment in my LIFE. A part of me died that night. And now, I get really worried because someday Choc will die, and seriously, I cannot even think about that.. I will lose it.