Why does my dog constantly tear her skin open on her back?

She is 2 years old. i treated her with antibiotics for skin infection on her back. I gave her vitamins. I changed her diet. I have given her steroids. I am given her spot on treatments. I give her antihistaminss everyday. I bathe her at least once every two weeks. she is an outside farm dog. the vet bills are adding up. what else can I do for the dog. Now she has another slight skin infection. I caught her today walking under the horse trailer back and fourth several times I went outside to see what she was doing and she was tearing her hair and her skin right off her back. I treated her with Iodine and gave her MORE anithistamisn and a steroid. this is insane. what do I do next?