When we are anxious, many physiological changes happen in the body providing us with the capabilities needed to fight or run away from whatever it is we have perceived as threatening. One of these physiological changes is our breathing becoming more rapid to provide the body with more oxygen.
Whilst this is a helpful response when faced with a physical threat that we need to fight or run away from, it's not so helpful when stuck indoors being relatively inactive as it creates an imbalance of oxygen and carbon dioxide in the body. This imbalance, by the way, is why some people feel dizzy and light-headed when they feel anxious.
Breathing exercises help us to take slower and deeper breaths, rebalance the levels of oxygen and carbon dioxide in the body and activate the body's calming system, bringing a sense of peace and calm.
There are many different breathing exercises and the trick is to try a few until you find one that you find helpful.
Here is one that I've been using recently to feel instantaneously calmer in the moment:
Find a comfortable place to sit. Slowly inhale until your lungs are comfortably full, counting the length of the breath. Focus on taking slow, deep breaths that fill up the belly like a balloon first, followed by the ribs and chest. At the end of the inhalation, take a moment and then exhale slowly for two more counts than the inhalation, letting the chest and ribs deflate and bringing the belly button in to the core so that all of the air is exhaled. For example, inhaling for a count of 7 and exhaling for a count of 9. Repeat this between 5 and 10 times.
Check with your doctor before practising any of the breath retention exercises. Breath retention exercises are not suitable if you are pregnant, have a heart condition or certain other physical conditions.