You can do it. You will do it. 
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Not sure if sharing my experience will help, or not but here goes because I do really feel for you, OP. When I was your age it helped me to write down everything I felt. Then highlight it all and hit delete. It was cathartic for me to get it all out and then just watch it get erased. That was my little space to say how I felt safely, free of judgment or retaliation. But once you hit that delete button, it’s over and gone – you gotta let the ugly emotions and feelings go.
Let that shit go. It does us no good holding onto it. You cannot control what other people do or what other people think. You can only control how you react.
When I was in counseling, my therapist asked me if I’d ever made a sandcastle as a kid and watched the tide or a kid come in and ruin it. Well, sure. She asked me if it was smart to fight the tide or even put my castles by the tide. No, of course not. “Well, put your sandcastle somewhere else… or, let the tide wash over you and just rebuild. You’re a grain of sand, the tide doesn’t matter to you.” That really stuck with me in a way that “let it go” never did.
Dysfunction perpetuates in negative feedback loops. It will enmeshed you both in the same toxic, dysfunctional cycle. For a lot of people, it is impossible to break because of generational trauma, mental health barricades, stressors, poverty, poor health and/or living conditions… It doesn’t make you or your mom a bad person. You’re just in survival mode. We’re not thinking more than five seconds ahead. We’re only thinking about how we can survive by fighting, or flight. And this permanently changes how we behave (if we don’t do something about it). Every interaction is colored by how we are only trying to survive and get out of a dysfunctional, effed up situation. Think of how genuinely awful and dysfunctional something needs to be for you to feel that your ONLY recourse is to take off a horse’s bridle… and think of how genuinely dysfunctional something needs to be for your mom to grab you by your hair, push you, and run away. That is a toxic feedback loop, that is survival at its worse, and will happen again if you don’t break it.
So break it. Every person has body language. You can read a room before you ever walk into it. Learn to see when your mom is having a bad day/mood, because that will transfer to you. Can you avoid her that day? If so, do it. And if you can’t avoid her that day, learn how to get out in a way that doesn’t provoke reprisal - maybe she needs something at the grocery store… Maybe you need to go to the library to study. Or, if you can’t leave, I bet there’s a fence at the opposite side of the property very far from your mom that needs fixing… During those bad days, do not retaliate when her anger is directed to you. You’ve seen ducks in water. Let that roll of your back.
Learn to see when you’re having a bad day too, because it’s not just about your mom. It’s about you, too. When you’re having a bad day, don’t even engage. There are days I come home from work so burnt out I don’t even interact with my horse because I know that “bad energy” will transfer.
And as impossible as it seems in this moment, learn to emphasize with your mom and show you are emphasizing with her. Just think: what awful things must have happened to her, with her parents, to make her this way? Nothing takes the wind out of a controlling person’s sails like not fighting back. With hyper-controlling people, they’re not escalating their behavior so you can be subdued: they’re escalating their behavior to get a reaction out of you, because they are expecting retaliation and they’ve already drafted their reprisal. When you are feeling cornered, take a huge breath. Count to five. Ask yourself if what is being asked of you will matter in ten years? Two? If not, just do it. Chances are the things your mother is trying to control you over are stupid and arbitrary, like where you put the leadropes or how you coil up the hose. So what? The moment you don’t give them that reaction is the moment they lose power over you. This element may be difficult to avoid if your horse is on your parent’s bill; it may be for the health of your relationship and self, that you give up Mercy and/or remove the horse element from your situation.
And… finally… learn to walk away. I can’t speak for your family or your experience, I can only speak of mine. I walked away for a few years by moving across the country. I had no money, I lived below the poverty line for years, but I made it happen. There was so much of the world I hadn’t experienced, and living in a new place without the support of my family gave me some hard truths and cold perspective. And I learned that I did really value my parents, even if my relationship with them was rocky when I left. Eventually, I came back home. It wasn’t easy in the beginning - it was downright awful, actually. But, there were several factors I could control. I could control my own reactions, and if I felt that I was having a bad day, or my mom was having a bad day, then it was better to leave and/or not show up at all. Feel that you are on edge? Leave, go grocery shopping, take a walk in the park, whatever. Just get yourself out of that situation before it blows.
It might surprise you to learn that now my parents are my best friends. I see both of them every day. Every now and then, things can get a little tense and when they do, I just take a big breath and let it go. It doesn’t matter, whatever it is, it isn’t more important than my mental health, my parents’ mental health, or our future.
And just remember, when you feel your back is up again… Repeat over and over: It takes two people to fight, it takes two people to fight, it takes two people to fight… Good luck. 
