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You must read Rabbi Zidni ‘Ilmaa. for brain power

Other than prayers, we need to be practical too. When you are not in touch with studies or are one of those who grab their books a week before exams, it becomes difficult to study because your brains are rusty. They are not used
to cope with so much study at once.

You need to lubricate your brain by creating a daily routine of reading and learning something. I believe a 20 minutes daily reading or learning is enough to keep your mind active, but this has to be done with the presence of mind. 

Always be regular with your assignments.

Lastly, be a positive thinker. It is a pshychological law that what is repetitive to the mind becomes a part of your character. If you believe your mind is weak, it will get weaker. Believe that it is a powerful centre of memory, all you is a spiritual cure, you have to put in practical effort too. Develop a regular need to do is lubricate it by reading, writing and learning, Insha Allah! The du’a and punctual routine of studies and always believe