I know Debbie gave you the link to the poem... but this is my favorite version. It takes a while to load, but very much worth it. Be prepared to really bawl though. Its much more powerful than the poem alone.
Rainbow Bridge

Its hard to loose a little one. I foster kittens, and have had a few that were very sick. I think I'd die if any of them ever died. I can't imagine how much you hurt right now.