This will be unpopular, but I think, from what you wrote, particularly about the sire and littermates, that your dog's aggression is genetic, and you should probably euthanize the dog.
"... but I am exhausted. I live in darkness, I dread walking because it is like walking an annoyed bear and I can no longer leave my dogs to even go to the shop (when the attack happened I had been gone for 20 minutes)…"
You got a dog for a companion, to improve your health, for mental/emotional health, and whatever else. It isn't working. I know, that he is a living creature and he's not like a Northern Light that you turn on for a few hours and if it makes you feel better you do it every day, but if it doesn't it can go in the closet or drawer. I know a living creature is a different story.
But you are living in darkness. I am worried about you. The liability to you, yourself, to you, financially, to you emotionally, to the children around you.
Yes, yes, you can crate your dogs when you go out to shop. Won't kill them to be crated or kenneled when you aren't there. And you can rehome your collie -- we might want to rehome the one with serious problems but if we choose to keep a dangerous dog alive, it is up to us to take care of it, so even if the collie was there first, even if the collie is easier, even if deep down you know you love the collie more (if you do, I don't know, but even if), the one to rehome is the collie.
And even if you rehome you collie, you may still fine it necessary to put the GSD down.
The thing is, this is NOT normal behavior for a GSD. What you witnessed with your GSD and the rabbit -- that's normal for a dog raised with other pets. A GSD should be able to live with another dog (ok bitches living in close quarters might fight with each other and they may fight to the death, and that may be normal). But we aren't talking about Same Sex Aggression in bitches. For one thing, in SSA in bitches it is almost always confined to bitches who live together. That same bitch can go to training classes, walks, parades, dog shows -- no problem. Usually, bitch-bitch aggression can be really bad, but managed by crating one when the other is out, or watching resources (like you) and managing those situations. You get really good at reading your bitch and acting before the bitches act/react.
But I am not hearing SSA/bitch aggression here. I am hearing a dog that cant stand to be in the vicinity of any other dog. I am guessing that this is getting progressively worse, he attacked you collie, he will attack the collie if you walk them together and you see another dog (re-directed aggression). People who do dog-training as their thing are being injured around other dogs by your dog. Little children may be trampled or worse if another dog breathes within a quarter of a mile of your dog.
If a dog trainer dislocates their hand, that's bad, but they sign up for this, this is their bag so to speak. If your dog turns on a child (redirected aggression), because he cannot get to the dog outside, it is safe to say that you will feel 5000 times worse than you will feel if you put your dog down.
You can spend 3 more years, thousands of dollars on trainers and tools, you can change your entire lifestyle, and still have to put the dog down. And, it won't be any easier.
This is what happens when you put your dog down: You go to the vet at the appointed hour, or they come to your home (some will). The find a vein and give the dog a strong sedative. The dog relaxes and falls into a deep sleep. Then they push the stuff that will shut down the organs. They will listen to the heart and let you know that he is gone. There may be a shudder or reflex, but often there is not. Physically it is the most humane way a creature can die. Dying in one's sleep of old age might be better, but that is really rare. In nature most death is really not very humane at all.
And then you walk away and tell yourself, as many times as you need to, that your dog is not suffering now. He had an illness/condition and was not right. You loved him very much, but you did the right thing.
There are a lot of dogs being put down because they are too big, or because they are black or because they have more energy than their owner expected, or because their owner died, or because they did not match the new furniture, or because they chewed on the new furniture. If you want, go and get one of these and give it all the love you have, because you have an incredible amount of love within your for critters, and it would be a **** shame for this one dog to have removed for you the joy of dog ownership.
Or, if you want, enjoy your collie and let it be an only for a while. Try something new with him, Rally, Agility, Herding. And let your collie help you to heal your heart.
Or, go and get a puppy from a breeder. You know some of the things to look for in breeders, the dam, the puppies, etc. These cases are pretty rare, actually. So the chances of a repeat of this are pretty slim.
I am sorry you are going through this. It is sad. And whatever you do, if you choose to try another trainer, some other tools like e-collars, crating and rotating, or building a good solid kennel system. Well that's your decision and no one should fault you for that either. Sometimes we have to try until there is just nothing to try left.
Heidi Ho, Odie
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Cujo2, Karma Chameleon
Ramona the Pest, Kojak -- who loves you baby?
Tiny Tinnie, Susie's Uzzi, Kaiah -- The Baby Monster.