You sound smart to me, I bet you can do it. :] You just have to have time management. I think with a schedule it will be easy. If you set aside a certain amount of time for schoolwork each day, and you really focus during it, you can spend all your free time with your horse if you want. And that way, you won't feel like you're neglecting either one.
One good thing you could do (and I was absolutely horrendous at this, to the point that I chucked mine) is use an "agenda book" as they called them at my school. We got one at the beginning of each year, there was a spot for each day, and we would write down all of the assignments for our subjects that needed to be done right when the teacher assigned each one. This is just a great way of doing it, because if you do homework at school and at home, you can keep track of which ones you finished easily and check them off as you complete them. You'll never forget to do an assignment or to study for a quiz/exam, because it will be staring you in the face every time you open that book. Even though I just could not be bothered with them, it would have saved me forgetting to do homework all the time.