try to focus on understanding of the subject rather than competative attitude. you know the solution process of problem it suggests that you study well and know the steps for solution. you might have anxiety to solve the same problem in classroom and you think more about others that time. just need to train your mind regarding performing in class with a complete focus and you need to be more confident regrding whatever you do. believe in yourself..give value to performance you and be happy that you attempt to answer in class.. Wish you all the best!

There are two problems here..first, lack of concentration, second lack of confidence. Whether one is reinforcing other, will determine best strategy to handle this. Don't put yourself under pressure, just convince yourself to work on the now. Practice healthy lifestyle like walk, meditation, good sleep and eating habits and engage yourself socially to grow confidence. Also seek help from a psychologist to help yourself better. Good luck.