I've christened myself the "angel of death" at my house because I am the one that has to make and carry out the decision. Yes, my husband supports the decision but he is to tender hearted to actually follow through.
So recently our kitty of I think 17 years was losing ground with her hyperthyroidism that we had her on meds for over a year. She was very frail and was vomiting often. I took a good look at her, not through eyes that saw the "kitten" in her (you know what I mean) and saw a very elderly and suffering kitty. I stayed with her and brought her back to bury next to the Great Dane she so loved many years ago.
Both our dogs faced the end with me holding them. It was the final act of a loving and caring owner (hate that word) could do for them. Our great dane Merlin went very peacefully, he had been ready for at least 5 months. Our newfie/retriever mix Bear was another story. Although in great pain and probably riddled with cancer (didn't bother test, according to the symptoms and advanced age vet said it was probably that), he fought to live. Tore me apart, but it was the right thing to do. Once I got his attention and cradled him in my lap, he gave in and stopped fighting. So typical of retrievers, puppies till the end.
I will have to face this decision with my horses, T is 20 and losing sight, but I will watch her closely and if she is always tense and nervous, losing weight, and without any zest in her life, I'll know we've reached that part of our journey together.
We can't be selfish and keep them going for only our sakes. Medically we can keep them going for longer than nature has deemed proper. Tough, yes, but they and what's best for them should always win out in the decision making.