How to make my rescued dog feel more comfortable in our home.?

We just rescued a white boxer named Sadie. She is very mild mannered but obviously previously abused. She cowers when you come near her or try to pet her. It's really sad. No matter how often we take her outside she potties inside and it's starting to stress our other dog out. Whenever she does something wrong our old dog barks and cries for our attention then she just leaves like I really need my space I don't want to be around her. If we catch her in the act we correct her behavior and take her outside and praise her for doing what she's supposed to do. I'm worried that she will be unhappy with us. She doesn't seem happy. What should I do?