Ok well ,I can understand your problem don't worry,

 Be strong with the concept which you already knew don't loose a single marks from those  concept,try to gain confidence with all your 12th syllabus even this helps you for your 12th board exam .

Best thing is you can go through with all the previous year paper it will surely help you. Don't loose your confidence

I hope this will help you

All the best

I suggest you to have clear understanding on the subject that you are really strong at and based on that you can choose the discipline that you can. There are many options other than sciences like even a normal degree has more value than the engineering program. You can also look for some aviation academy course to be a pilot. CA is also an option by opting MEC. You can also opt for visscom. You can even be into gaming &graphics industry.