It's always a judgement call with a pet but I try to consider, above all else, what is right for the animal. Not my own feelings.
We lost a cat a while back, Mr Squeak. The vet wanted me to give him daily injections, possibly twice daily and to undertake an exploratory operation when we were 90% sure it was a tumour. He hated just having a pill - how he'd react to injections was easy to know. Poor chap would have thought we were torturing him.
Life has to have value to be worth it - whilst Charlie was happy it was right to let her enjoy it. The cure wasn't working and I believe that carrying on a distressing treatment that was very unlikely to succeed was the wrong call.