have you tried putting hydrocortizone on it? You can get it from walgreens and cvs. that's what we used to put on our horses when they got rashes or muck itch and it helps a lot bc it's good for healing like neosporin but it also stops the itching. Our vet was the one who prescribed it a long time ago and its for people but she said it was a lot cheaper than the horse version and it works just as well and isnt harmful to the horses.
"The horse you get off is not the same as the horse you got on; it is your job as a rider to ensure that as often as possible the change is for the better."