Practise breathing into your belly. When we get anxious we start to breathe faster and shallower. That means that our breathing is in our upper chest and not in our belly. How do you know where your breath goes? Start with putting your hand on your belly and notice if you belly is actually moving in and out when you are breathing. If it is not moving, or it is moving very little your breathing is too shallow.
What do you need to do to change it? Imagine you are breathing into your belly or even better your pelvic area. Notice if this makes a difference. If not maybe imagine you have a balloon in your belly that inflates when you are breathing in and deflates when you are breathing out.Keep doing this until you feel your belly moving under your hands. Then relax into this breathing for a minute or two and notice how you feel.