Hi there!

Well it is not only your problem, everyone in this world has the same problem that you are facing.

It is in the human nature that they starts forgetting things over a period of time, not only in the field of study but also things in the regular lifestyle.

If you are facing that problem while your studying, don't worry you will definitely overcome it, I have been facing these same problem when I was preparing for entrance exam.

From my perspective if you follow these certain steps, it will definitely workout for you.

1- First and foremost whatever subject or topic you are studying make sure you are giving your 110% on it, human often remember things which they like most, hence you should be committed to studies,

2- Whichever topic you are reading, keep a habit of writing along with it, possibly you could make small notes of it, because mostly we remember the things which we had written.

3- Practice more and more, if your are dealing with numerical you have to practice as much as you can.

4- And the final and most important REVISE every Saturdays and Sundays whatever topic you had covered in that particular week, here the small notes which you had make will come into role.

These are some basic things which will enhance your studies and it will feel enthusiastic all time.

I hope it will be useful for you!