I have only felt once that it was definitely the right decision at the right time. That was a dog that was very injured in a two on one dog attack. The last one, I definitely waited too long. It was only a week, but what a horrible thing to put an animal I loved through. The decision is personal, difficult, and many times does not feel right. We do our best and love the next one that purrs or wags into our life.
I just hope you find peace when the time comes to make the decision for your current furry friend.