I used to have the worst panic attacks in school, still get them today actually(but not as often), and I can tell you what worked for me. Each person is different and handles their anxiety in their own special way. I tried to focus on breathing exercises but that did nothing for me. What I found helped me were two things actually. First to calm me down I recited Shakespeare in my head. My go to was Romeo and Juliet or Macbeth. Once I was calm I would focus on a song and sing that in my head until the hesrt palpitations stopped. I found that the iambic pentameter and the rhythm of the music helped me focus in a way that breathing couldnt. If you're struggling to find something for you you can always try the Calm App, or see a guidance counselor/social worker. They usually have good suggestions. You may even go as far as talking to a special education teacher. They thrive in the field of anxiety.