If you do surrender her to a shelter, identify her as a Boxer cross, not as a Pit. You do not "know" that she is a Pit, she just looks like one, as do any number of mixed breeds. If you surrender her as a Pit, you make her one, which can affect her adoptability. Many insurance companies will not insure your home if you have a Pit, but will if you have a mixed breed. She has enough problems as it is, don't give her another.