My new mare is just miserable with itching!!!

She was bad last summer when I bought her, but I thought I had it licked and tried to prevent it again this year. Wrong. Does anyone have any tried and true fixes??? I'm feeding her vitamin C and E, Sprulina, Apple Cider Vinegar and fly spray each and every morning. She is still really itchy, from her midline down. HELP!!!
...and God took a handful of southerly wind, blew his breath over it and created the horse...
~ Bedouin Legend