You sound very intelligent and determined. There are plenty of guides on preparing for the SAT available. I'll bet you can get one and teach yourself. I don't know what's available in the way of online support for people preparing for the SAT, but I'll bet it's there.

You can take the SAT more than once, too, if you need to. (I'm sure you know that, just saying.)

If there's a college you would like to go to, you might contact their admissions office to see if they have advice for home-schooled students.

Best of luck to you!