very dog aggressive small dog

[sportygee]

I inherited my Pomeranian/chihauhua mix from my aunt who passed away las year. I took care of both of them for a year and when she was nearing her end I promised I would always care for her dog. Up until then it was just my Aunt and the dog and neither of them were being properly cared for. I knew the dog had some training that was way over due and took it upon myself to get her to be more dog and less a baby as my aunt treated her. She quite happy to assume the role of dog and me as alpha. I kept my promise and even when I moved with my fiance from Califormia to Washington State us. She is very attached to both of us but she is definitely more my dog.
Mitsy was not very well trained when I arrived on the scene. My Aunt also had her on an extremely bad diet. She was also on medication for seizures. She was over weight and skittish an completely unsocialized to humans and other animals.
I taught her to walk on leash, to come when called and her diet was changed completely. Between the change in diet and daily walks she went from a very chubby 13 lbs down to a proper 9lbs. She is also completely free of seizures and off all meds.
She is also now people friendly in controlled situations like when we are on walks she likes children to pet her and she does not nip at people. She does bark like a nut when someone comes to the door but she doesn't charge them. It is more on the line of letting me know someone is at the door. Once I open it she goes behind me and stops barking on command.
My biggest problem and one I am very concerned about and can't seem to fix is her dog aggression. We live in a very large complex with lots of dogs. She also hates cats but she is not aggressive with them. She takes at least a 30-45 min walk with me in the morning. She then has about 15-20 minutes in the evening for her to do her evening. She is fed once a day and she divvies it up over the course of the day when she is hungry. She is completely housebroken and taken out every few hours to handle her business. So I think she is getting the right amount of excersize and stimulation but she acts absolutely psycho when she sees another dog. She barks, growls, lunges and were she not on leash would charge and start a fight. She was constantly going after my Sister's poor 14 yr old pure bred mini poodle when I took her on vacation to visit family back East.
I have tried walking her in another direction to redirect her focus. I have tried to introduce her to other dogs in controlled situation. However nothing is calming her down to the point where I can trust her off leash in a dog park or anything like that.
I made a horrible mistake today. I was sweeping my front entryway and I had the door open. Then a neighbor brought her two dogs out on leash across the parking lot. Before I heard them Mitsy heard them and shot out the door past me and charged them. The quickly put her in check and she ended up backed under a truck while they went on about their business. My dog started the melee but the other two dogs finished it. No damage and no bites to any of the dogs. I know it was my fault as she was not on leash and escaped from my apartment. This could have been a mess as she did not stop when I called her and was on a mission to start a fight. Once she was put in her place by the other 2 dogs she took off from me and would not come on command for a bit. I finally got her and went to pick her up and brought her home.
I am at a loss as to how to change her dog aggression as I can't tell if she just hates other dogs, is scared of them, trying to protect me or just a product of 4 years of no real contact with the outside world. In the year she has been my dog she has learned quite a bit and gets out all the time. I just don't know what to do about her behavior towards other dogs. I will be quite careful in the future but I want her to see another dog as a possible friend and playmate instead of something she is supposed to try and beat up. Any suggestions would be greatly appreciated.
Gwen