Poor Belle. As Glacier posted, it just takes time. Star went through the same thing when our Rosie was hit by a car too. She was soooo depressed. I started taking her everywhere with me and spending as much time as I could with her. That seemed to help. She didn't eat for a while either.
(((hugs))), what a tragic loss.

*something else that Star did that might help Belle too, was that she carried around one of the toys she & Rosie played with together. It seemed very comforting to her. She still has it.