When I was a kid, I had a beloved Collie that was my constant companion. He got older and was not feeling good for a while. He just disappeared one day and we never found him. Typical country dog. He knew it was his time and he left to die with dignity.
Had to put one down a few years back. One of the hardest decisions I have ever had to make. I know that it was the right one, but it didn't make it any easier for me. Good luck, wheels. You will do the right thing when it is time.