First of all, do not think anything negative. Whenever you sit to solve any question, then think that you can do it properly.
Spend time with people who have a positive effect. Do your regular prayers. Prayer will help you increase confidence reduce your stress as well.
Also, to be confident while doing math is to understand the concepts properly and memorise all the formulas by heart.
Keep all unimportant issues at bay. Be focused. Stay away from distractions that could cause loss of concentration or unwanted anxiety. Stick to activities that do not break your study continuum.
Revise all the ncert questions and examples properly and then practice the sample papers and past year paper available on our site.
Avoid calculation mistakes by practicing and by writing the solutions step by step.
You can then definitely score very good marks.
All the best!