Updated: Jun 3, 2020
I am in no way a psychologist or therapist, but having gone through high school myself, I know how insanely stressful it is. Many studies have shown that stress (and lack of sleep!) negatively affect your performance, including your grades. Don't forget to get enough sleep, do some yoga, and give yourself credit for your hard work.
One book in particular I loved is How to bean Imperfectionist by Stephen Guise. I was the definition of a perfectionist, which led me to procrastinate and spiral every time I got a B on a test. The book encourages you to adopt the growth mindset, where every misstep is progress and a lesson learned. Check it out!