Thank you for writing in.
I found that the best part of your message is "I score quite good." This clearly shows that you are overall a good scorer but there must be something that is stopping you to be motivated and confident to score and prepare equally great in class 12 board exams.
Try to figure out the reason - why is it happening, since when it is happening, was there some incident or health issue that led to this situation. Understand what will happen if you do not score well in these exams. Realize your goals and visualize where you want to see yourself in the coming years.
A goal setting process might help you in this case. We would be happy to have conversations around it with you.