Some people scoff at the notion of grieving for a pet, but for owners who were close to their dogs, the pain is as sharp and lasting as for a family member.
The short answer is - grief is a process with no short-cuts, it has to be gone through and it wil take its own time.
There will be days of intense sadness, and days of happy memories, but you need to rolll with your feelings.
The worst aspect is other people, who will assume within a few weeks that you are over your loss, and fail to understand if you are still upset.
Try and understand that this is the way a lot of people perceive pets - except those who love and loose them, so be strong, and you will feel better as the days pass.
Eventually you will think of your friend fondly, and with a twinge of pain, but the good memories will long outlast this pain.