Made that same hard decision in November for my 18 year-old dachshund-spaniel mix. She had suffered a stroke, and was reduced to walking around in left-hand circles, and had no bladder or bowel control. Tore me up, especially when the goodnight injection caused her to go into wild spasms in my arms before she shut down.
Until they build a dog that can live as long as humans, we'll always be saddled with that harsh reality.
For Missy and I, it boiled down to a quality of life question vs. my selfishness to keep her near me as she suffered.
My two big dogs are now 4 years old. I will dread that day when it comes, but it will.
My heart's with you, Mike.
