I don't have advice, we're in a similar position.
Our cocker spaniel is 15.5 years old and struggling. She's deaf and her eyesight is terrible now. She's not off her food or drink, and doesn't appear to be in any discomfort, but she also doesn't seem to have any joy in life any more, and I think she has dementia (she doesn't really know who we are ). She's been an amazing loving pet, she's never so much as shown her teeth at anyone, despite years of having her fur pulled by toddlers etc. Now she's not easy to look after (regular ear and eye cleaning, and she goes to the toilet inside more than out), but I don't want to put her down just because it would be easier for us.
Ahhh Trig you have my deepest sympathy. I said in my post that we sometimes let them go on too long as we can't bring ourselves to say goodbye. I wrote that because I think we did that with one of our cockers (which are easily my favourite breed). I never minded all the cleaning up and the fact I had to carry him everywhere, I just wanted to keep him with us. He still wagged his tail and he still ate his food and I thought that was enough. Eventually my wife, who loved him just as much but was stronger than me, talked to the vet and they agreed it was time. 'Devastated' doesn't begin to describe it.