Difficulty in memorizing 12std subject.

You could have scored high marks in 10th std and now less than 50%. Depends on the study methodology you adopted till 10th or maybe the school trained you to memorise for the sake of marks and did not train to UNDERSTAND the subjects. It is time to adopt different studying methodology as memorising will no longer work. Dont get medicine dependent,  it has its own disadvantages and side effects.  To boost confidence exercise, meditate,  hit a gym etc.  You need motivation from your parents and family members than finding fault in you. If still in 12th grade you have 3 or 4 months to work on the methodology and bounce back to high mark scorer yet master the subject.  Begin from where you can begin and gradually progress for humongous achievement

Consulting a Counsellor who can help in Study Methodology will help

don't do for medication without doctor's advise.