Same problem
We are going through the exact same thing with Rock, we lost Bonnie just last week & Rock is so sad & bewildered. He won't let us out of his sight, I think he is afraid we are going to go away too, it really is heartbreaking. I now think I made the wrong decision in not taking him with us to say goodbye to Bonnie. He has never been an only dog so it is very distressing for him to be on his own now.
I also think that like Rock senses with us Luna will be sensing that you are very unhappy, if you can & I know how hard this is try to be happy & playful around Luna.
We take Rock out where we know there will be other dogs for him to play with & he seems happier then, maybe you could try this with Luna, I think this will help in the long run. Good luck xxx