So, I was the first student to go to college in my family. My little brothers followed. For me, I couldn't have a job in HIGH SCHOOL unless it was summers--until senior year-- and in college my parents cringed at the idea of working a part-time job! This is because in their mind, education WAS a full-time job that was only compromised by distractions like work. Work means less time to study, do your homework, get involved in research or your area of interest... And not that this is untrue! But to give you perspective, I did work a part-time on campus job in college, and now both of my brothers in college have part-time jobs on the side. ;) It's do-able if you're willing to work hard and balance your priorities.
Now, I didn't own a horse in college, so I can see how panicking about working enough part-time hours to cover board might interfere with your classes. But, can't you get an on-campus job [to solve the car problem and the "focus on school" problem all at once]? Most on-campus jobs (IME) are essentially jobs where you can work and study at the same time (library monitors, gym monitors, etc). I was literally paid to sit in a hall and monitor the racquetball courts & rock wall while doing my math homework. One summer I also worked in a research lab which paid quite well.
The second thing I will say is that yes, college is harder than highschool. I was an easy 4.0 in high school and found myself challenged when I went to college; I had to work harder to earn those grades. It does take more time and effort.
When all else fails... I agree with finding a "free lease" for your horse, where the leasee pays cost of board, feed, vet, farrier (etc) until the lease contract is up. This will likely be your best bet.
Just a few other thoughts... does your college have an equestrian team? A friend of mine and I were on the college equestrian team and she received a discount for boarding her horse at the barn that hosted our team. You might even be able to set something up where they use your horse for team lessons, etc, to pay part of the cost of board.
A horse is so much responsibility and involvement that I cannot imagine owning one during my freshman year of college. You have a new lifestyle, new friends and social experiences, your education... college is a fun, life-changing experience! It might be to your benefit to go into college with "no strings attached" and no financial obligations to tie you down from a night out with your friends or an all-nighter at the library.