Ours was like that too. Fought the cancer for 18 months. Eventually it made him stop eating, and we had to make the decision. And while we knew it was happening, and took time off work to spend a day or two with him at the end, the change in the environment when he wasn't there anymore was still jarring. I've done it a few times too, but we always had another dog in the house. It was never quiet and there was still one to greet you when you came home. The stuffed animal kind of gave us something to say hi to when we came home, and hold if we were sad. We weren't ready to get another right away, and this really helped.
Good to hear you will rescue another. I'm sure you'll give it a great home and life.