Nursing an abused budgie back to health

I gave my brother my parakeet because I was no longer able to care for her. But, today I came to check on her but she was ridiculously underweight, her nails were waaay too long, and she is completely traumatized. My brother had neglected her to the point of abuse and I was heartbroken. So, I took her back and I’m trying to nurse her back to health but she has terrible trust issues. She freaks out whenever I walk in the room or change her food and water. I want to get her in a good mental state as well as physical state but it’s so hard when I don’t know how to gain her trust. Does anyone have any tips for helping me bond with her when she is terrified of me? I really want her to be happy and healthy. Thanks!

Edit: also she’s about 3-4 years old if that matters.