My dog is extremely aggressive:please help!!

About 6 years ago I adopted from a woman a 2 month old German shepherd hound mix whom I named Abby. She was absolutely perfect, until around the age of 2 years she started acting extremely dominate and hostile towards other dogs. After a couple of fights with the neighborhood dogs she gained a reputation and I avoided other dogs she wasn't familiar with. Dogs that she was familiar with were very submissive and let her be boss. After a couple of years without incident I began walking without confrontation. It started with the whining, then the growling, and now a nonstop terrifying booming bark the scares people. I corrected her as soon as she begins but its almost like she cant hear or see anything but the dog she has targeted, I have tried pinch collars, choke chains, harnesses, muzzles, dog trainers who tell me they cant do anything because they cant figure out what is making her freak out. I took her to dog parks which worked for awhile then she began acting aggressive again. She wont even smell dogs anymore, its kill than ask questions later. I adopted a Jack Russel to see if it would help, it did kinda. She adores Bella and puts up with how annoying she is. People have said "show her who's boss" from day one its been that way. She is completely submissive to me, so much so if I say leave it alone or don't eat that she wont touch it or stay and don't move she wont she wont enter the kitchen or walk through a door without my say so. She shows me her belly when shes in trouble and is never disrespectful to me in a sense. She is also very gentle with my cats. Please can anyone shed some light on what I can do to help her, I hate her wanting to kill every dog that she sees. Thank you.

It's time to break out the checkbook and hire a professional trainer to address her behavior.
   
This will be expensive but worth it as she is one incident away from a lot of trouble.