Been there!
A little over a year ago, I had to make that final decision for our beloved 10 year old rottweiler, Shelby.
She was with us through thick and thin and had to stay with my parents for 6 months because we moved from Ontario, Canada to DFW, Texas in May and it was too hot for her to fly, so we had to wait 6 months for the airline to ok her flying and she was put in cargo and flew by herself. She never lost her laid back attitude or her love for us.
As she got older, she had arthritis in her hips and some hip displyasia, she slowly got worse. A year ago all of a sudden within 3 days she went from climbing a flight of stairs to sleep on my boys bed to not being able to move her hind end. She could not get around by herself and she looked at us as much to say, "this isn't me, and I don't want to be a burden".
Taking her to the vets that last time and coming home without her was the hardest thing I ever had to do. Our 4 kids were devastated, as were my husband and myself. But I believe she was better off. She died with dignity and my utmost respect. (You see since we moved here, I haven't been alwed to work, due to my lack of formal education and eligibility for my own work visa, but that's another story, and Shelby was my constant companion day in and day out when everyone else was at work or at school. She would follow me from kitchen to computer to bedroom to mailbox. Wherever I was so was she.) I know I did the right thing and even now I get tears thinking about it, but she went quick and peacefully and I was by her side till the very end.
Peace be with you in whatever you decide for your constant companion.