How Do I Get Through This?

My little dog went missing over a year ago -- I believe he was either shot or stolen. I've done everything humanly possible to find out what happened to him. This has been an obsession and I just can't get past it. He is constantly on my mind. I've lost pets in the past due to disease or old age. It was always tough to deal with, but at least I could say goodbye and know they were not suffering. I'm tormented by the thought that he could be mistreated. There are so many books about dealing with the death of a pet, but none that I can find about lost pets. How do I find closure on this?????