You've got many tough things to deal with, way more than I ever had. That said, I think you're being tough on yourself too, almost certainly a consequence of living with your N-Mom's craziness. So I'll speak to school stuff, because for me school has been the launching pad to having a good adult life.

First of all, a B- is NOT barely passing. But even if you failed a required course, the answer is to take a break and then try again, maybe from a different program or teacher.

You started your post with, "I've been struggling with math," and I totally had that experience in high school. After flunking some Algebra tests, I was happy I passed the course with a C! So later in life I re-took Algebra and found out I was awesome at math, almost certainly because my brain had matured (but I still had to study hard--it never came "easy"). We've all got our own rhythm to life which may or may not conform to how schools present subject matter.

You wrote, "I don't want to work hard if I know I'm gonna fail or be mediocre." All humans will fail from time to time and we are mediocre at most things. Life is more than the number of subjects listed on a report card! Success is almost always more a function of hard work and grit than genius.

Another "failed" experience in my life was taking three years of piano lessons. I was never going to be Mozart or Little Richard or even the Nordstrom pianist, but at least I can read music and have a keen appreciation for those musicians who ARE great. And they need me in the audience or buying their music or else they'd be "failures" too.

Be kind to yourself even if your mother isn't. My guess is that the more time you can get out of the house and away from N-Mom, the better you'll feel about yourself and your studies won't seem so formidable. Thomas Edison said, "Genius is one percent inspiration and 99 percent perspiration." Your motivation should be that you're preparing to have an awesome adult life! Hang in there.