Sorry for your loss dude, I've never had cats (they don't like me and I'm allergic) but I remember when our two white alsatian twins died many years ago now, the brother of a lung tumor and the sister a year and a bit later of a heart attack - both congenital problems along with some digestive problems.
Both times it felt like a sharp kick to my gut since they died so young (I think the siste only reached 5) and I was quite young at the time aswell.
It sucks to lose a pet, especially one who's been part of the family for so long.
