This may be totally irrelevant, since my experience was with rabbits and not dogs, but it is food for thought.
I had three bunnies, two females and one male (all spayed and neutered!!). The females never really got along very well, but one of them really bonded with the male, and the two of them were like peanut butter and jam.
I had to euthanize my big bunny this past fall, and my little girl was inconsolable. It was awful. I had hoped she would find comfort in the other remaining bunn, but she was just aggressive to her. I did not want another rabbit because there is no vet who really knows rabbits where I am now living, and I felt that I had as many animals as I could manage both financially and in regards to care.
But I could not stand my my little rabbit's grief. My own grief was particularly intense due to many of the circumstances surrounding this death, but I think I have shed even more tears over her loss.
I finally broke down and got her another boyfriend. He was a rescue bunny who had been in an excellent foster home, and the introductions went well. My grieving bunny is a much different rabbit since the death of her buddy -- grouchier, yet cuddlier, too -- but she has done MUCH better since the introduction of the new one. She is even getting along better with the other female.
Sounds like your situation might be similar, but with a different species. And if anybody ever tries to convince me that animals a) do not have feelings, b) do not grieve, or c) do not remember, then you know where they can go! In any event, another dog might be something for you to consider.
I send my heartfelt condolences your way. I know firsthand what a helpless feeling it is to witness a beloved pet's grief on top of your own. It is hard. I send you many comforting thoughts and the hope for a good resolution to your problem.
p.s. My rabbits and my dog do NOT mix... The dog is willing, and would be most gentle with them, but the bunnies will have none of it.