+1 on considering leasing her. Or even getting a half-lease.
I'm a college professor, and have sent two children off to college (one has graduated, the other is to graduate next May). I've seen a lot of the kind of stress that attaches to this life transition. It's not an easy time, not for anyone, and for some people, it is very difficult! You are moving into an unfamiliar surrounding (possibly moving for the first time ever), you don't know what to expect from your college classes, you are going to be meeting a lot of new people (and may be wondering whether they will be people you like), you'll be taking your leave of your parent's house and your friends, at least for a little while. This can all be very scary! Even if it is something you're really looking forward to, it can be unsettling. It is totally normal to be a little freaked out about all of these changes. It is also pretty normal to be more than a little freaked out by them.
There may be something else going on for you - I can't tell - but if there is, you might want to consider talking to a counselor or a therapist, or maybe an older friend of your family, relative, etc. about what is going on with you and for you. I am sure that there is someone in your life who can be supportive and offer you a little guidance as you go through this.
Now, to the nuts & bolts... I can tell you - as someone who worked my way through college, and who has many students who are doing the same - it is definitely possible to work 20 hours per week and take 15 credits, and still have some time left over for socializing and having a life. The key is having good organizational skills. This book
is one I recommend to all of my advisees, and I gave it to my stepson as well. It's full of very realistic, very good advice on how to make sure that the time you are spending on your classes is getting used well, and that you're not going to get behind on your work. It's a pretty easy read. The term after I bought it for my stepson, he brought his GPA up by a full half-point. Now he's on the Dean's List, even though he has a significant learning disability. So, with good time-management skills, it will be possible to work 20 hrs per week and still do well in your classes.
Next, your horse. You've got to ask yourself how much having your horse around contributes positively to your mental state. If the horse helps to stabilize you, and helps you feel better about yourself and everything else, then that is something that would be working for
you, in very important ways! It may be that you would be a better student and have a more rewarding college career with your horse on hand, even with the need to work off her keep. Also, if you have any tendency to go into a hibernation state from emotional distress, ask yourself whether the obligation to keep your horse groomed up will encourage you to get out of your cave, or whether it would feel like a crushing burden and make you be more in your cave.
If you're going to a horsey school (sounds like it, if there is an equestrian team) it may be that there are already people around there who would be happy to get into a lease. Even a half-lease would cut down on the amount of time you need to spend working off her board, and it would also cut down on the need to exercise her as frequently. I would definitely suggest pursuing this - it's not going to cost you much to sound people out and explore the options.
Last - that issue of needing to exercise the horse. I cannot tell you how many of my students wind up crying in my office because they are so stressed out about all the work they are having to do, etc. - and then when I start asking them, it turns out that they're spending so much time with their books that they are not getting any exercise. This is a disaster!! Holing up with nothing but work is the second worst thing you can do as a student (the worst is to not work at all :)). Whether you take your horse or not, you need to budget in an hour or so every day of physical activity - this is the thing that is going to recharge your batteries so that you can actually get something out of your studying. Way I feel about it - and duh, I'm on this forum - is that one hour with the horse is worth 10 hours inside at a gym, for what it does for my body, health, and mental attitude.
Hang in there, know that you are not alone in finding this transition to be daunting, and definitely consider leasing your girl.