I am in the same boat! My mare is 22 and has become worse over time. She chews her belly raw if she gets one tiny sore that the flies cause. I have done heavy worming after reading about neck threadworms and it seemed to help at first, but now back to ridiculous itching. She breaks out in bumps too here and there. I have tried supplements with dried apple cider vinegar and yeast, herbs, shampoos, fly sheets, enviroment changes, coconut oil, ointments, etc. My next option is the vet again, but a different one and possible allergy shots. The flax does help a bit too. I get the stuff at Costco that is milled flaxseed. No heat used in processing. Not a cure all though. Let me know what your vet says. Thanks!