Dont panic at this stage. you have studied for the past 6 months and they wouldnt be gone. Revision will help you remember them back. Start with past papers, solve them. If you make mistakes then revise till you get them right. There are still 3 months to go and you can score decently. It is the fear that harms more than the actual exam. What you can also do is take a chapter, revise all related questions from past year papers so you feel strong. Make short notes to keep track of what you seem to forget so that you can easily revise them at the last minute. 

Most important - remember its just an exam and you can do it. Dont compare, dont think what ifs. That will not help. Just study and revise.