Another one here . He's nearly 16 and has arthritis, and is senile. HE also often collapses and cannot get back up without help, and sometimes whines and wanders round aimlessley at night.
I went through it 3 years ago too with my first dog and also with my 2 old lady cats.
I belive now, that you just 'know' when the time is right. There becomes no more 'is it time/isn't it'..you just know.
One of the deciding factors for me is, is there still a quality of life? My old boy loves his food, still comes in the car for a daily wander round the park. Sometimes he even sniffs around the odd tree but never does this in the garden. He will not walk on the lead, hates it and needs to go slow at his own pace, hence only the car to the park now.
Would your dog be able to go in the car to the park to have a wander like in the garden?
I still think that when its time you will feel differently to how you feel now, with this indecision, but I don't know you or the dog, so have a little look at this Dogs Trust guide for some more information :
www.dogstrust.org.uk/information/atozofdogs/b/bereavement/timetoletgo/