I have once come as close as actually making an appointment with the vet to have the dog euthanized, but fortunately it never came to that.
The dog came from a kill shelter death row and we were fostering him for about 6 hours.. when we took him for a walk, something my boyfriend did triggered him and the dog literally tried to kill him. My boyfriend ended up in the ER with half a dozen puncture wounds (they seeped for about 2 months), missing piece of flesh on his arm, a tooth that was pulled out from his inner elbow, millimeters from major blood vessel (and the worst of all, pretty significant PTSD)... I was able to grab the dogs ankle and hold on while my bf managed to get to safety.. I'm not sure what would've happened if I didn't manage to grab a hold of him.. the dog was eerily quiet through the whole thing, determined to bring him down on the ground and go for his throat. There is a good chance that if my bf wasn't a big muscular guy this would not have ended well..
As I coordinated things with the rescue group (who technically still owned the dog) to get him euthanized, honestly, I was very concerned for my own well being. I just had to put a dog down several weeks prior (he was almost 13 dying of cancer), I was in a very bad state and I tried to save another dog in order to deal with loss. Having that end with putting another dog down was going to be tough, but I didn't see any other options. Then the other owner of the rescue group called and said that she feels the dog hasn't been given a chance, because he wasn't decompressed properly (mostly because we didn't know what decompression was and no one told us, so not something I would argue with) and that she wants me to take the dog to a rehab place 7 hours away, which I ended up doing. I have no idea what happened to him, since the rescue group stopped talking to me shortly after.
I grew up in Europe with a pretty aggressive airedale who bit a number of people and dogs -- nothing that came close to the incident described above, but once he bit a 12 year old girl, because he didn't like her dog and she needed to go to the hospital. That was tough. We brought the dog with us to the US when we came here. He lived till he was almost 13 and died of cancer. We had several incidents in the US, but he mellowed out dramatically as he got older. Having that dog was a lot of work, but where I came from aggression wasn't considered abnormal and it wasn't bred out of dogs, so it wasn't that uncommon. The thought of rehoming him or euthanizing him has literally never ever crossed my mind.
I now have a VERY fearful dog, who requires so much work that it made me feel like I know nothing about dogs even though I've had them most of my life. He has never bitten a person, but I put a lot of effort to not put him into a situation where he might feel like he needs to. He is part doberman and barks very intensely at all visitors and strangers. It takes him probably dozens of interactions with people who are good with dogs in order to become somewhat comfortable. He does not warm up at all to people who are not respectful of his boundaries and limitations.
What helps him greatly is de-stressing daily, which for him means going off leash and running through the woods. I go out early in the morning or late at night to places where no one else goes so that he can do his thing. He also has a number of dog friends who he sees regularly. They make up his social life, because he can't really go anywhere where there are people or strange dogs. It's not ideal, but given his personality (he was not at all socialized most likely and was considered un-adoptable by the shelter he came from and was about the be sent back south) he has a pretty good life.
I would say that having a fearful anxious dog has greatly increased my own anxiety, since when I take him out I'm constantly on alert. There are a lot of things I can no longer do, places I can no longer go to, people I no longer hang out with because of the dog. A HUGE part of it is learning how you, yourself, need to behave in order to convey to your dog that you are in charge of the situation, you will protect him, you are his fearless leader and you will take care of the situation so that he doesn't have to. It's not easy, but I feel like it's a worthy goal. Another big thing is accepting his limitations, which is also not easy and something I struggle with, but I do believe that's a big part of finding peace.
You may need to find very knowledgeable people to help you.. and as others said, regular dog trainers have no idea how to deal with aggression. There are, however, some that do and they may be worth searching for.
Of course what you do is up to you and what I'm saying may not relevant at all. I come from a different culture, where
a dog is not really a pet, but your companion and your dependent. I also feel that, as St. Exupery says in "The Little Prince" -- you are forever responsible for what you have tamed.