I know this pain
You knew this day would come, as I know it will come for me, again, as well.
Having dealt with this pain myself once, I can tell you, nothing hurts more. I can also tell you that after this, after knowing the immeasurable pain, I got another dog, knowing that I would have to someday endure it again.
So, rather than mourn the loss, celebrate the life.
Fortunately, your dog passed quickly, without suffering the months or years of growing old.
For me, and hopefully for you it was a great adventure, something you can treasure with memories for a long time. Your were lucky to have this experience.
I'm sorry for your loss