Hi Ray and welcome. You've been given so much fabulous info, not sure that I have anything more to add. Only more words of encouragement. I also have a dog-aggressive dog. I have (well, HAD) the German Shepherd version of yours. I know I'm her 3rd home, not including the shelter or the rescue org (so does that count as 5?) I had that terrifying German Shepherd who'd spot another dog a block away, and who'd be up on her back legs, snarling, barking, lunging, etc - totally terrifying to everyone (including me - cuz I kept thinking .. what if she breaks lose?). I knew I couldn't go it alone, and found the most fabulous positive-reinforment training facility. We went thru what's called "Feisty Fido" classes - classes for aggressive dogs and their owners. It was the best thing we ever could have done. She'll never be a dog-park dog, but we take wonderful walks in the neighborhood, at the beach, etc. I'm telling you this to give you hope. Please don't think this is your life. It's not.
Where are you? If we knew where you were (exactly), perhaps we could help you find a training facility or trainer similir to the one I went to, who offeres a similar type of class. In it you'd learn to read Harry's body language, how to manage all kinds of different situations, different behaviors to teach Harry which will help him out in the world, etc. In the class, dogs are also worked with trainers and other dogs, all learning how to cope in the presence of other dogs and humans (since some dogs have people issues, some dogs have both). All dogs are worked "under threshhold" - meaning, under that level at which they aggress. In the class, dogs aren't stressed to the point of aggressing. The whole point of the class is to give people the knowledge to manage their dogs, and set dogs on the path to learn how to cope in the world.
Let us know where you are, and we'll see what we can do. And you - take a deep breath. Help is on the way!!!