Well my girl is 4 and for 2-3 years I've been going through the same. Faith has a real dominance issue, she's not one to back down first and being that we are both as stubborn as each other we've had our fair share of barneys.
She's like your boy, leathal from both ends. I had my chiro out treating another mare, and Faith was getting nosey, so he gently pushed her away, no more than a touch on the shoulder, she promptly turned around and double barrelled him in the thigh
She was always worse at feed time. She'd try and bite when I was within reach of her mouth and she'd kickout if I was behind.
My solution, now this will upset/anger some people and for that I'm sorry, I'm not saying 'do this' I'm just saying that it helped in my situation. Please don't condemn me
If you watch horses having an argument in the paddock, the winner is the one who gets in the last kick/bite. I was loosing my battle until someone mentioned this to me, so how did I have the 'last laugh' without getting my head bashed.bitten off?.........
I met up with Poly! Who became my best friend and ally for a few weeks.
I hear you say "huh!??"
Poly is a 2-3ft long piece of 3inch poly pipe, the type used in watering systems. If Faith EVER
threatened me, I laid into her! Now she'd come back at me, but as long as I got in the last 'kick' I won! After she'd stop we'd have cuddles to reinforce the good (her stopping)
Now for those who are ready to black list my name forever, try hitting yourself with it first. Poly doesn't really hurt a human, so it sure didn't hurt Faith. It does however make a very 'angry' noise.
After a week or two, I just had to carry Poly and Faith knew. Now all she'll do is flick her ears at me.
So for me it worked, I will never be alpha mare, but we are on equal footing now and have a much calmer relationship.
Sorry about the size of this novel!