I'm in the same situation, except right now i'm leasing. Just a part lease though; I can't afford a full-lease either at the moment. What I would suggest is to find a way to make money. I've learned how to braid and am braiding for John Flint at the Midwest Horse Fair. Sure, i'm doing it as a favor to my instructor, but it's a great way to get out there and really figure out what i'll need to be doing.
I really want to show this summer (and show in Maclay classes in high school) so i'm planning on baby-sitting and working at my friends' parents' store because we can't afford it if I don't find some way of making money.
I don't know if you do this or not, but if you have lessons on the weekends, can you stay out all day? I do that, and I usually end up helping my instructor, watching other lessons, and doing odd-jobs around the barn to help out, all the while learning. At this age and in my situation there isn't much more that I can do, but you should try to do as much as possible, even just to help out or watch lessons or make money doing non-horsey things to support your horsey passion :). That's what i'm doing, and things have only been getting better since I started doing these things.
And one more thing: have you asked your trainer if you could do a part-lease on the horse? Tell her that you simply aren't financially able to do a full-lease, and if even if she says no, at least you tried. I'm sure she wouldn't hold it against you :). There's no harm in asking.
And, above all, make sure you don't let horses get in the way of your school work. For a little while I was doing that, and my grades dropped to C's and low B's. Your parents will (most likely) restrict your horse time if you aren't keeping up in school, and an education is the most important thing. Good luck!