Dear candidate, don't worry many students face this problem of forgetting what they studied. To conquer this you can increase your studying time. Actually this happens because students try to grab more syllabus in very less time and they end up forgetting. Increasing the studying time will give you more time and you'll be able to study as per your requirements.
Also, one more thing you can do is to practice what you have read by writing that. I implemented this technique. When I was in 12th and had hundreds of Chemical reactions to remember, I write them on regular basis. Some were logical and you can frame them but some were needed to be remembered so writing them on regular basis helped alot. Try to study whole week and at the end of the week write all the stuff that you have studied. If you think you just forgot write again until you are confident.
All the best
Hope this helps