Please don’t worry about this in 8th grade! (Or any grade.) No college will see 8th grade grades, but more importantly, worrying about a college application 5 years before you apply isn’t good for your mental health. Do your best on that test and that is more than enough. It’s important to care about your future but not at the expense of your present.
You’ve got this. Getting an B in math is not a failure—not by any definition of the word. And your future is going to be very bright.