Adding some sort of calmer won't do much of anything until you get rid of the source of the horse's angst ---------the feed
Liken it to a diabetic loading up on two dozen cupcakes, then wondering why the meds to control their insulin doesn't work
I have a horse in similar vein to what you describe. He is oat/corn and SOY intolerant. Soy is his worst enemy.
I stopped needing the riding crop in the bath bucket within two weeks of taking him off grain & soy products. I might add I had been arguing with him about 13 years of the 15 I had owned him, when I took him off grain/soy.
I feed him a ration balancer than is grain and soy-free. If your horse is a hard keeper, this may not work as it is designed to control weight. McCauley'sŪ M10 Balancer
While the change in his personality was remarkable, what really helped turn him into the horse, on a daily basis, that I only used to catch glimpses of was Remission.
I suspect he is one of those horses that is magnesium deficient. He is on one enclosed scoop daily until Spring grass comes on, then he will receive two scoops daily.
This horse has been with me 17+ years and I am still kicking myself for not figuring this out years ago
His personality changed so much for the better, even the vet asked if Rusty was sick when he gave Rusty his physical last June
Hope this helps:)