I went through this. It's really tough. With my Annie, it was little things at first. Like you said, Walking and dropping poo, Poo accidents in the house, trouble getting up and down, sleeping a lot. My daughter graduated high school in the spring of 2011. Annie was doing OK then. Nothing too dramatic. A year later, she had little to no muscle mass in her rear. She could no longer do steps. I often had to lift her to her feet. She drug her rear foot badly. Sometimes, she would fall. Her quality of life was failing. By last summer 2012, she was 14 years old. Her hearing and vision were poor. She often seemed confused. I knew it was just a matter of time before she lost complete use of her rear legs. One day, she wouldn't be able to get up any more. I didn't want to be in that position of taking her to an E-Vet to be put down. I had the vet come to my home and she went peacefully surrounded by her family. That was last August.
It is a very hard decision. Like you, I did not believe her to be in pain. That made me hold off on the decision. It was the continual decline that pushed me to make the decision. I know it was the right one and I was at peace with it. If you aren't ready to say, "Good-bye", you don't have to be in a rush. Realistically, you know your girl isn't going to get better, only worse. Only you can decide when the time is right.
Just know that I am only one of many who has going through this. We are all here to support you. DM is a cruel affliction. My heart aches for you.
Natty Boh 6-27-12
Annie 1998 - 8-2-12 RIP