I know you will feel bad, but please feel better in that you have given her a chance to find a home that can cope with her. If you had kept her, like I said in my answer, you may have got into a situation that would have forced you to have her put to sleep. Believe me, if she had savaged another dog badly (possibly killed it) and then you HAD to take her to be put to sleep you would feel far worse than you do now. She is probably not beyond help, she just needs someone far more experienced than you are. I have had dogs that are fighters and believe me it is no fun, you have to be constantly on your guard, not just for your dog but for other dogs who come running over to your dog to 'play'. It really is more trouble than it is worth.
Please don't let this experience put you off owning a dog, please go back to the home, or another rescue centre (explain what has happened to you) and believe me you will find a dog more suited to you. You sound like you have alot of love to give and given the right dog you can learn together and have fun together, letting them off to play and run without constantly looking around, afraid of what they might do.
You made the right decision for everyone including the dog, you have given her the chance of a good life with someone who understands and can cope with her needs.
Good luck for the future, and I hope you find a loving and faithful companion!
Thank you for letting us know what happened, I was just thinking of you today wondering how you were getting along.