I am an ambitious fool, a daughter of da Vinci
Smart people aren't obligated to go to college and become a doctor or lawyer or some other high-powered career. Smart people aren't even obligated to finish high school. Not doing those things doesn't magically make you not smart.
People with bad grades aren't obligated to improve their grades at all cost. Grades do not determine knowledge, wisdom, or character. Lacking knowledge of some subjects doesn't mean you're not brilliant.
Embrace what you have, even if no one else thinks it's enough -- prove them wrong.