At what point do you know when is best to put your dog to sleep?

My dog is having problems with his hind legs. He is now on medication to see if he will ever recover the use of his hind leg. One is affected and he has trouble getting up.
I hate the thought of putting him down. He has been my companion for almost 14 years. He will be that age on May 8th.
I dont want him to suffer any more but at the same time--he seems ok except for this inability to move up to go anywhere.
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I’m having problems just lifting him to go out, even using a harness to lift him. I’m alone and can’t do it. I have for a few days but it’s clumsy and I always feel as though he will get hurt, so I have towels under him to catch his urine. He barks to go out, and I feel terrible.
At what point do you know when is best to put your dog to sleep?
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