Hi, BW. I had that talk with my mother, just after I turned 18. And yes, she gave me the boot for not believing. She even had me DF a few years later.
Bide your time, and you're not baptized, right? Maybe they will let you stay at home, if you keep a low profile about your issues with the religion.
You're being home-schooled, if I recall? Are you getting the proper curriculum, and will you graduate? If I were you, I would focus on getting the most out of my schoolwork that I could right now, and staying underneath the radar with the religion. Try not to get mom too worked up.
Now, the possibility of being 18, and on your own. It can be done, and especially if you are healthy, and determined, you'll be okay. You also have the support of this community, which is HUGE.
Don't live in fear of what may happen. Prepare for the worst-case scenario, and hope for the best. Be realistic, and know that you have the right to your own life. The happiness of your mother is NOT your responsibility - you are your OWN PERSON, and she has no right to selfishly live her dreams through you.
I'm happy you came to the forum for support - you can do this! xx tal