How can chanting a mantra help me handle exam stress and panic attacks?

People usually call me Karan Mehta. I was born in Pune at 5:15 AM on 5th January 2001. Every time I have to sit for my exams, I get such a bad feeling in my chest as if it is being crushed, a lot of odd thoughts enter my mind, and my hands get sweaty. I came to know that chanting repeatedly like "So Hum" or "Om Namah Shivaya" really helps in one's relaxation. During my examination breaks, I will be very short ones. If maybe you have some time before the exam or during the breaks, could you please show me the correct way of taking the breaks? Would it really help to alleviate the anxiety?