Maine coon/ siamese mix anxiety

We rescued our cat from the shelter. We have had her for a month. She is 3 yrs old.  I need help. She is terrified to death of everything. She hides in the bathroom, so I made her a spot in there. She will only come out at night when we go to bed. The way i understand it she belonged to a older woman and she had to give her up. I can tell she must not have brushed her. She has alot of matting on her stomach and back side.  I pick her up and love on her and try to brush her. But she is looking the whole time and trying to get down and run. She is terrified of ALL noises. I have done everything I know to do. I need someone to tell me/ help me . I'm getting so discouraged.  I want her to be part of the family. Not a cat that hides till we go to bed and I hear her walking around. Thank you. 

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