My best guess is your horse is allergic to someone's bite. It could be Midge flies, mosquitoes, even plain old house flies.
If she's like that on her chest, have you checked her belly line and teats? She could be bit all to heck in those areas too.
And the insides of the ears ------ I'll bet she's bit up inside her ears. You can use any kind of healing ointment and rub it gently inside her ears. It will heal the bites, plus the ointment acts as a deterrent against future fly bites.
It is truth that diet helps deflect insect bites. Sweet feed is the absolute worst as it permeates thru the skin when the horse sweats --- the odor is terrible if ones nose is sensitive enough to pick up on it:(
You could buy SWAT and slather it on her chest but that is big area to cover with SWAT and it would have to be cleaned off every day.
I am no help with fly spray, except to say Zonk! Works really well the few times I do need fly spray.; I still have a half gallon from two years ago. My horses are both easy keepers so I encourage them to go in the barn, lollol
My barn is set up to where each horse has their own entrance and their own drum fan. I leave hay for them and they come into the barn whenever they want.
If you don't have the option of putting her up during the day with a fan, I have heard decent reviews about Absorbine Ultrashield. https://www.valleyvet.com/ct_detail....FQODaQodeegIgQ
Or maybe someone will come in with an all natural fly spray recipe.