Bonding with birds takes time, and the hardest thing to remember is to have patience. Based on the fact that her previous owner gave her up to get a cockatoo, she may not have spent her first years in the greatest care. It really seems like you care and you're doing everything right for the situation you're in.
Firstly, although it seems counterintuitive, for now you may want to only let her out of her cage for an hour or two at a time. She's just gone through a major life change, and chances are that her cage is probably one of the only places she feels safe and comfortable. Let her stay in her cage but leave the door to the cage open (with your supervision of course) and try to let her come out on her own terms. Hopefully, she'll soon want to start exploring and you can gradually up the time she's out of the cage so that she learns to enjoy time out of the cage rather than fear it. If the cage she's being kept in currently is different than the one her previous owners kept her in, you may need to start with only one or two toys. Let her get comfortable with those toys, and after a week, you can add another one. Wait another week, another toy, etc. Along with this, it's how you're spending the time out of the cage that matters. If she has nothing to do out of the cage and no toys and no interaction, she's obviously going to want to be in her cage. Two hours out of the cage with constant or near-constant interaction is better than eight hours out of the cage with nothing to do. It's all about engaging the bird.
Second, check what's in her cage. Sometimes excessive biting can be a sign of nesting behavior. Check the cage for nesting materials such as cotton and shredded paper. Also check for something like a sleeping hut or a mirror as these can also cause nesting behavior. If these things are in it, remove them, as they can cause cage aggression and more biting, and possibly egg binding, which can be fatal.
As a tip for now, try getting her to be more comfortable with stepping up. Check your hand position. Is your finger steady or is it shaking? You should be at a 90 degree angle rather than parallel to the ground. If your gcc still seems afraid, try teaching her to step up onto a towel first. This way, the bird has a steadier place to step up onto, and you avoid getting bitten so much.
I hope this helped! Remember it's going to take patience, and birds as a species like to get what they want, and can be very sassy little creatures, but that's why we love them.