help me fix my aggressive dog

[sportygee]

I have a pomeranian/chihauhua mix and I don't know why but I cannot stop her aggressive behavior towards other dogs.
I inherited her from my Aunt who I took care of for a year before she died. The dog was 4 when I got her and she was not trained in anyway except housebroken. She was also overweight and she was treated like a baby and not like a dog.
I fixed those issues, got her leash trained and people friendly. She gets 2 good walks a day not including her trips out to handle her business. Her weight went from 13lbs down to 9lbs and she is now off the seizure meds she was on when my Aunt had her. Vet believes her poor diet was the cause of her seizures.
I just cannot let her off leash in any situation and we live in a complex that has lots of dogs and cats about. Whenever we are on walks and she sees a dog she lunges, barks and growls and is ready for a fight regardless of the behavior of the other dogs. To be honest the only dog that has exhibited any aggression towards her is a pitbull that lives in the complex and he goes after all the dogs here.
I have tried controlled dog meetings when she is on leash but that is not working. She escaped once, my fault but the 2 dogs she went after put her in her place and she was forced to back off and back down.
Any suggestions as to what I need to do or what I am doing wrong. I know some of this is lack of socialization from pup to 4 yr old but I have never had a dog with an issue like this and am at a loss.
Thanks