How can help a rescued 13 yr. old abused Chihuahua. She tries to bite me when when I pet her.?

She screams and tries to bite me when i try to pick he up. If you try to touch her neck - putting on a collar. A child can pick her up and pet her with no problem. Once I et her in my arms she is okay but still scared of being touched on her neck. sometimes put her in a sling and walk around with her - she is hapy then, as long as I have her in my arms. All her teeth were covered with plaque. After eating hard food all except for the back teeth fell out. She will eat some food , then run away from the bowl and then come back for another bite. She's not spayed and will tun her back to male dogs. When I first got her she seemed to have her period every three weeks. How can I help her overcome the abuse she endourded for years?