It is a crappy situation! I hate that we are having to even consider putting him down. We have rescued many of animals himself inclued as a small pup.
Unfortunately there are not any shelters or facility that has been willing to take him. Due to the fact he has harmed other dogs mostly.
I wish I could have found some but our area is very rural and even the bigger towns don't have much resources in that aspect.
We have a 4 year old female Catahoula. She came into our lives after a seriously abusive "home". At a certain point her "resource guarding" behavior and self protective actions caused injury to one of our older dogs. We had two older dogs at the time, both also rescues but neither had the intense issues this pup did.
We had been in touch with a Dog Behaviorist at a Shelter 75 miles away. He had given us good advice regarding managing her behavior.
When the dog's behavior escalated so dramatically, we made an appointment, took all three dogs to visit the professional.
He assessed the situation and gave us this: The Catahoula had been removed from her litter before the age of socialization - probably around 4 weeks. She had never learned limits from her litter mates and mother. She was in a constant state of anxiety and could not make decisions. We all have that line where we can snap from exhaustion or fear or whatever, this dog lived at that line 24/7. It did not take much to push her over the edge.
She could not be rehomed or surrendered to a shelter, for obvious reasons.
He suggested we ask our Vet for medications. The Vet agreed.
Three years later, she is a sweet tempered dog, protective of "her" chickens and kittens and is now learning to play with the new pup in our household (a Border Collie who came to us at the right age, and out of a healthy situation. Both our older dogs died of illness not injury). She is great with visiting children although she wants to herd them back to the adult group - but she heels them and does not nip. She is not always great with visiting dogs - they seem to want to play with her kittens or show too much interest in the chickens and she is very protective. We have to run interference and set limits on how our guests handle their dogs while here.
Honestly, the meds (Zanax and Prozac) saved this dog's life. We would have had to euthanize her otherwise. She still could never be rehomed, or live in town or surrendered to a shelter. This is her home. Had she ever escalated to killing, we would have had to make the other decision.
We are fortunate in that this is not a dumping ground for unwanted pets, we do not have strays or close neighbors. Her territory is clearly defined with the electric in ground fence and she defends it...
I am so sorry you are dealing with what sounds like a similar but more serious situation. Follow your heart and intuition on this. It sounds like your dog is very similar to our Catahoula and just couldn't handle the big life changes... It also sounds like you've done your best.
Personally, I'd rather decide it was time to euthanize a dog with serious problems than have a neighbor or guest, or an injury to a person, force the issue.
good luck!