That's rough. Our pets are family, and it seems likewise for you. I've had to put down a few over the years, and end up an emotional wreck each time. The worst isn't when it happens. I've sat there at the vet, weeping of course, but remaining composed enough to hold my buddy and give him/her comfort and security during those last breaths. Then, after they go, I find solace in knowing their suffering is gone, and a blissful transference of anguish upon myself. Their time is done, and yet they live on in the reveries of my mind. The hard part is coming home, no happy scratching at the door, no barks or wimpers... only silence. The most agonizing silence I've ever incurred.
RIP to your buddy. He may be physically gone, but his influence will remain, for as long there are memories in your mind, he is never truly gone.