Tough decision. I certainly feel for you. I've always let my pet tell me when it's time. If my pet isn't interested in food and water, it's time to go. As long as I could medicate and treat the symptoms of whatever they had, and the pet was still eating and drinking, I thought it was all good.
If you're green, you're growing...if you're ripe, you're rotting!
I try to greet the world like my pets do...boundless enthusiasm, intense curiosity and no concept of yesterday and tomorrow.