It can almost feel like a cliché to be told to focus on your breathing when you are anxious or panicky. However, this is one of the most straightforward things you can do to bring yourself back into a calm mental state. When your trauma is triggered, your body goes into fight/flight mode and starts trying to conserve oxygen, so focusing on slow, deep breaths communicates to your brain that everything is OK and there is no danger. You can find out more about trauma and how it impacts your brain through this YouTube video.
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As well as using breathing techniques to bring yourself back into a state of calm when you are feeling anxious or triggered, you can also use regular breathwork and meditation to expand your window of tolerance and reduce your anxiety overall. There are many resources that you can use, and this video is just one example. Explore the options and find what is right for you!