I study for my chemistry exam and starting think what if after I study so hard like I did in my first exam and in the end I did bad what if I did this time too what if the exam come so hard and idk how to answer

always I want to be an engineer and build houses and be like my dead uncle but after thus chemistry exam I realized that I will never be like him and never be what I want to be. this is so sad that I study so hard then all I did is vanish like nothing happened this is so sad and so hard. I want to stop trying but I can't and Ik that