Well, if the behaviorists don't see any hope in your ability to manage her, you really don't have many options.
Since she is fine with you, you could take her somewhere and become a hermit. Maine is a nice place, and there is plenty of semi-wilderness for you to live. Â
However, that is probably not an option.Â
 This really is a terrible decision. Euthanizing her seems to be necessary, since even behaviorists have recommended it. Â
 Passing poor Petra on to someone else's problem doesn't seem like a good idea, unless you can find a behaviorist to take her. They would have enough knowledge to keep anyone from getting hurt, but others might not know what they are getting into with a large and aggressive dog.
It is sad to put an animal down, but that is better than seeing her attack someone. Now, THAT would be truly heart-breaking.
Sofia