My dog is very sad/stressed over finding a dead mouse. How do I help her get over it?

My dog Pepper found a dead mouse in the garden earlier today, ever since then she has been pacing, barking, whining and stress yawning/licking. Nothing we have tried has been able to distract her, and as far as I can tell the mouse is the only cause for this stress, I asked her if she was feeling sad or hurt and she chose sad (she knows how to pick between 2 options and how to identify her own emotions, so I trust that her stress isn't being caused by any sort of pain). I said to her "Pepper sad, mouse hurt?" And she ran over and sat down in front of me, which is her usual indication for “yes”, so I'm sure thats the reason for her acting this way. This behavior is very odd for her because even though she absolutely loves animals, she understands the concept of death and has never been bothered by it before. We have had several pets that Pepper was close to that died (2 rats, 1 rabbit) so she knows the word "dead" and that it means she can't see that animal any more. We also always point out dead animals that we see on walks to reinforce the concept. When she first found the mouse in the bushes, she picked it up and brought it over to my mum. Pepper was whining and licking it as if she was asking for us to help it. My mum took the mouse away and told Pepper that it was dead, which she usually understands and accepts, but for some reason this time it really upset her. The only other time I have seen her react like this was when my other dog Clover got surgery, and Pepper was very upset at the fact that Clover was in pain. Pepper is very empathetic, if I tell her I'm sad she will run up and lick my face until I smile. I should mention she's a border collie and therefore has a tendency to get fixated on things (especially to do with animals) so I just want to make sure this doesn't become an obsession. She has had an obsession before about a spider she saw and it took her a month to stop staring in the corner looking for it. We tried taking her mind off the mouse with play, training, a walk and hugs but she was just so distracted and distressed the whole time. She's in bed right now, so hopefully she will have forgotten about it by morning, but if she continues to fixate how can I help her get over it and feel better?