My very aggressive dog. I need help quickly!!!

[KRissam]

Hi,
I have a dog that is about a hundred pounds and some kind of mixed breed. I got her four years ago from a family who felt they didn't have enough time for her. SHe had clearly been tied on a short run for a long time. She is very very aggressive towards other dogs. I have tried taking her to trainers, obedience classes. We are at our wits end. SHe is the sweetest girl inside, she is even pretty good with kids. Though the last time there were two girls here she is comfortable with she was snapping at them. What do you do when you have a dog you love, but is dangerous toward other dogs. We can't walk her because if we meet another dog it is all I can do to keep her under control. Should we have her put down. I have already had a few injurys from her because of her aggression and she sent another dog in the neighborhood to the hospital and once she busted out of the back of the truck cap at the grocery store and attacked a man walking his dog. She actually was after the dog, but the man got in the way. I don't blame him. We do our best, but we live in an apartment building and another dog moved in upstairs. She is going crazy trying to get the other dog. She breaks windows. WE leave the curtains closed all day. Her prong collar on her with a leash so we can pull her off the window. She is really out of control. I am afraid she is going to get out and hurt another dog fatally or a person who tries to get in between them . It will break our hearts to put her down. I am completely against destroying an animal, that is why we rescued her, but is it right to put other people in danger because of our selfishenss. Are we being selfish? Help please.
Krissam