that sucks man, sorry :'(
we had to put down our cat last year out of the blue. he got real sick in a matter of a week and we took him to the vet expecting to get some medicine and he'd be all better but he had a massive tumor near his rear leg. it really sucked.
here's what I've learned about death from several funerals of relatives, it's like having a piece of you ripped out and the period of mourning is the time it takes for a scar to cover that wound, but it will never fully close. you might forget about them for a while, but it will always come back and you'll be reminded of those you've lost. then you'll forget again, maybe until someone else dies. rinse and repeat.