Zoom, I understand what you are going through - from my own dealings with beloved, elderly pets, to those clients of mine I counsel and help everyday.

Here is what I have my clients do that are starting to think of end of life issues:

Make a list of the 5 things that you think bring your pet comfort or quality of life. It can be as simple as being interested in food or a favorite place to sleep or coming to the door when you get home (a little more tricksy in kitties since they are so secretive about their favorite things sometimes).

Then decide how many things on this Quality of Life list must be gone before you euthanize. And one thing on the list can carry more weight than others. That way, when you get to the point that those 2 or 3 things are off the Top 5 list - the decision is made for you and there isn't any wavering because you have already made this decision at a time when emotion wasn't a factor.

I had my mom do this when our elderly corgi got sick and we knew "that day" would come. She still thanks me for it to this day - she was torn on whether to treat Penny's condition (acute on chronic renal failure) or whether to let her go and she looked at this list and decision was made, because she knew ahead of time that she had already made the rational choice when she could think clearly and was not a blubbering mess talking to me on the phone.

I hope that helps. If you need anything else, let me know.