While loving your dog is the greatest joy, losing them is the hardest sorrow to bare. There comes a point in the relationship you have with your pet that you have to decide whether your keeping them alive because you can’t deal with the loss of their company or your keeping them alive because they will still have a few wonderful quality years of life remaining.
Pain and Suffering
In answering this question, you really have to ask yourself if your dog is in pain and constant suffering. If so, then the humane thing to do is have him/her put down (or put to sleep) by your Vet.
Your Vet is the best source of advice about diagnosis, what treatments are available (If any) and how successful they are likely to be. Ultimately the Vet would base his/her decision on the quality of life that your dog could have if he/she were not put to sleep.