When we think the mind listens and the body reacts. There are always alternative ways to interpret your thoughts. Thoughts are not facts and once they pop into the mind they should be held accountable. Acknowledge your thought and choose to either accept or replace it. 
The next time you get a negative thought pop into your mind write it down and next to the thought write down an alternative positive thought. Even if you don't believe it you are teaching your mind that there are alternative ways to interpret your thoughts. Overtime this will reduce anxiety, panic and stress. Don't expect to see changes overnight. Like anything this method takes practice but once mastered the results are truely astonishing and life changing. I can speak first hand of how this method changed my life and I have heard many stories from mental health professionals how it has changed their clients lives also.