What should I do after an exam to alleviate the anxiety that afflicts me when I discover my mistakes in it, despite my diligence and prayers?

Everything that befalls a person, be it good or evil, is by the decree and divine destiny written in the Preserved Tablet. Belief in this brings peace of mind and tranquility to the soul, and increases faith. Furthermore, the Prophet, peace and blessings be upon him, advised to be keen on what benefits, to seek help from Allah, and to abandon saying "if only I had done such and such" when calamities strike. If worry and grief persist despite remembrance (dhikr) and supplication (du'a), then that is not from Allah, but rather from the servant himself; due to obstacles to answered prayers, such as lack of certainty, or heedlessness regarding the meaning of remembrance and supplication, for Allah does not answer the supplication of a heedless heart.