Time management is the key here. Assuming you don't have big topics to study now, I'd say focus on revision now. Make board exams a priority and revise for them. Approach with a relaxed mind. I know it's hard to be calm right now because of all the exams and stress, but keeping your head high will help you think better. Try to revise what you have already learnt instead of learning a new topic altogether. Solve some sample question papers to time yourself better. Now is the most appropriate time for that and most importantly, don't over think. After you are done with board exams, shift your focus towards other exams. Look over important formulas and important question types. Eat healthier and maybe walk a bit everyday. Approach the exams like they are exams and not some life altering phenomenon. Good luck.