Hello,  first of all think that,  why negative thoughts came in your mind. Most of the time the reason is same,  means if you are week in any point or you have not clear nay topic. So try to clear your concept and syllabus topics, so that you should not have any shout about syllabus topics.  Once you are ready with this negative thoughts will not affect your confidence. And still same thoughts came in your mind , then say your self that  " you can do it". And imagine the situation that you " cleared your jee main with good marks and you got your dream college ".

I hope it will help you.

thanks.