Gypsy you are sooooo wrong in your post. You did NOT fail yourself or Rosie. You did the very best by Rosie, gave her the most wonderful chance she could have in this life. Sometimes there is so much damage that the right thing to do feels like a failure but it isn't. It is never a failure to love another spirit and take a chance. It is never a failure to open your home and show love and compassion. No matter what the outcome here, you must know that you did right by Rosie. She isn't evil or bad, she just may be too messed up to help.
I went through sort of a similiar thing years ago, and it took me a very long time to move past it. I won't talk about it on the boards, but if you would like, you can email me and we can talk some privately.
Take care, you are a gentle wise person and the right path will show itself to you. As for not taking fosters again, I don't think fostering is for everyone, but you can't make a decision like that in the midst of your pain. Maybe after some time has passed you will feel that you can't do it again, or maybe you will have a different view and be able to do it with less emotional investment. I don't know, my hat is off to you because I know myself and I don't let go easily.
None of this makes you a bad person, and none of it makes you or Rosie a failure. Every happy loved moment she had where she wasn't biting was a moment of joy and success.
((hugs))
Ronda and Badger