Its hard but its most likely the right thing to do, unless some correctable issue is found. Thats pretty much in the range of a normal life span for large dog. Usually, there's something degenerating thats led to this behavior. I put two dogs down this year, one for physical, one for mental reasons. The one had CCD (canine cognative disorder), which is like alzheimers for dogs. It was tough, since she was physically in OK health for a 15 yr old dog, but quality of life wasn't there. She was nipping at people (everyone but me), confused, barking, pacing, etc. Its sometimes easy to miss signs, since they gradually grow. But usually, there's a reason. They've lost sight, or hearing, brain issues, pain issues, etc. Its a reaction usually to being in pain, confused and/or afraid.
Sorry for it all
Its the way of life, but it still sucks.