Please excuse me for kidnapping your post but you said nearly the exact thing I was going to write (if you substitute grandmother for father). It has been 4 years since I had to make that decision for Scarlett and it still haunts me.
At some level, I have to say that people in my neighborhood unwittingly made the pure fact of the decision easier for me. There was the shepherd who lost the use of his legs, so his owner rigged up a harness and a little red wagon for him to get out of the house and to the park; a chocolate lab whose owners brought her to the park in a jogging stroller for a year, even though she could only hobble a step or two. I know that they did what they did out of love for their companion animals. I also know that I have to be strong enough to make the decision without factoring myself into it. I have to choose what is right for my girls. I made that promise when I got Scarlett 19 years ago and I will stick to it.