You can keep thinking about what happened - the attack, the reactivity, the fight, the prey driven chase... But that will not help you to solve the problem.
I know you are scared. You have seen what your dog can do and you don’t want it to ever happen again. You don’t see a way out. You are terrified.
Even some dog trainers can become scared of a dog they work with, especially if they have no experience with such a dog. I get it.
But you are not alone.
Once you are scared of the dog you are working with, you should step back and let someone more competent handle the case for you. You will not be able to help this dog effectively.
To you, this dog is no longer a dog that can be helped; it has become a ticking time bomb, an unpredictable monster that will hurt you and other dogs/people any second.
You cannot possibly develop a mutually respectful and trusting relationship with this dog.
But it does not mean no one else can.
People often ask how can a dog trainer be so calm in front of an aggressive dog.
That calmness is not magical, it comes from confidence, and this confidence comes from experience - understanding of what is really going on, and the deep conviction that this will work because it has worked many times before.
Just because someone says a dog is hopeless does not mean it is the truth.
If you are scared of your dog, if you think your dog is really dangerous, very unpredictable, very aggressive for no reason...and if you are thinking of putting your dog down out of this fear - a fear based on things you have never seen and things that you don’t really understand - please take a step back, and seek help from someone else.
Please look for someone who is not afraid of the dog, someone who has done this many times with success, someone who specializes in this type of things.
You are not alone. Do not let your fear consume you. Give your dog a chance.