You know what the difference is between a dog and a human? Doctors generally don't euthanize humans so everything is about prolonging life and control is taken out of our hands, with a dog the control is in your hands, our hands as owners we are expected to make that massive decision we're not trained for it, we're not ready for it .
having had dogs for over 40 years and having said goodbye more often than I care to remember I can tell you it never gets any easier it never feels right you always feel awful you always feel guilty but at the same time allowing a dog or anyone you love to suffer is wrong and much more painful.
my old vet had a good way of telling you it was time , he would lay out all kinds of options and then ask ' who am I doing it for your pet because he or she will have a better quality of life if you're you because you can't say goodbye' ?
this is not the time to be selfish it's not the time to put your feelings first, this is the time to put them first , to make sure that they are passing is the most peaceful painless and dignified it can possibly be.