You know, the worst part is right now - watching him suffer and knowing that the decision is coming. It's god awful.
I think you'll find that once the deed is over, it's almost a relief. You never have to see him suffer again. He'll be at peace. You're not killing him, you're sending him off to paradise. It's totally up to you whether or not you go with when it happens. Don't think it's some nightmarish experience, though. The animal is sedated before the actual euthanasia drug is administered, then they simply drift away, as easily as falling asleep. Very peaceful and pain-free.
I'm really sorry for what you're going through. This part of pet ownership sucks beyond words. It'll be okay, though. *hugs*