That's the problem. They don't show pain like humans. My Rott "Sasha" was always a healthy beast and one day I noticed her shoulder looked dislocated. I massaged it and gave her some anit-inflammitory. Nothing chnaged and the "lump" got bigger. I grew to the size of a baseball in a matter of weeks. I took her to the vet and they diagnosed aggressive bone cancer. They said the only way to try and stop it was amputation. Problem was she was 9 years old and very big upfront. It would have been near impossible for her to walk again.
She didn't show many signs of pain but the vet warned that dogs don't show pain like people and that they would continure to push on even through pain. We gave her pain meds to alleviate suffering and it was ok for a while. Then the limping became worse. Even though she'd run and play, you could see the pain. I knew she would have to be put down.
It was THE HARDEST thing I have ever had to do. A few weeks later my Dobie "Suzie" just fell apart. It was almost overnight. She was about 15 and had lived a hard life as a stray until we got her. Putting her down was not as hard because she was visibly in pain and it was an easy decision to give her peace. Mind you, it was still hard but not as much as the Rott who looked and acted ok.
With those two gone my Chow mix "Musso" went into a depression (they mourn like people). I decided that I'd need to get him another companion and we went to the local no-kill shelter on Camp Pendelton. I found this shelter online and they showed pics and profiles of their dogs. I found a Rott mix that had been at the shelter for 4 months (one of the longest times for any of their dogs) Her name was "Tasha". I took Musso to the shelter to meet Tasha and they got along like they had know each other for years... So the decision was easy. Adopting from the shelter was great because she had all her shots and even a free tracking "chip" implanted.
I thought I was done with the 3 dog circus at this time. But as luck would have it, I was driving home one day and saw a beautiful golden dog (some shepherd mix) tied to a stop sign. He has a spike choker collar and the string was tied so close to the stop sign that he couldn't move. I notcied that some dogs were "harassing him" and there was nothing he could do to defend himself.
I stopped and looked for an owner. I checked and there was no ID collar. I cut his string and took him to my home where I let him stay in the garage. It was friday evening when I found him. My intent was to take him to the local no-kill shelter down the road. It would have to wait till Monday. Before the weekend was over the "boss" decided that we should keep him as well. He is the most loyal and gentle dog as well as very protective.
Sorry to hijack your thread but I can empathize with your situation and I want you to know that sometimes a hard decision is the right one. Check with the vet as well... As for a new companion, please check out the shelters. Give a deserving pooch a shot at a new life. I promise that whatever dog you get, they'll be super greatful.