Vet tech actually sounds like it could be a very good place for you to start. If you could get a job with a combo small/large animal vet, you would be able to gain experience around treating horses while bringing home a paycheck
I think the job this summer, GED, then community college is a very mature, organized, intelligently ordered set of goals. Also, once you get a two year degree under your belt, it wont matter if you graduated highschool with a diploma or a GED, when places ask for education you can list your degree.
In the meantime, there may be a little money to be made working at a stable if there are any in your area and that might allow you to earn some lessons or a little money while you get your horse fix