I totally feel your pain. We went through the same thing with Mimi last year. We finally let her go in Sept at age 15. The last year we had her was REALLY hard. She was wobbly, kept getting stuck places while we were at work, couldn't do the stairs, would keep me up all night doing this annoying breathing thing that I'm pretty sure she did on purpose
She was still eating and alert. We decided to let her go. It was a HORRIBLE decision, but we were afraid she was really going to hurt herself while we weren't home. It sucks when you're basically having to decide if they're just too old. She was pretty much healthy, so it wasn't like she was sick. We had our vet come to the house, so that made it easier. PM me if you want to talk about it. Kelly (Mimi was my heart dog, so it was REALLY hard to let her go)