What do trained pilots do when they face an emergency? They don't wing it (no pun intended!); they refer to their emergency checklists. Even with years of training, every pilot works through a checklist because, when in danger, sometimes it's hard to think clearly.
When kids face anxiety they feel the same way. Why not create a checklist so they have a step-by-step method to calm down? What do you want them to do when they first feel anxiety coming on? If breathing helps them, then the first step is to pause and breathe. Next, they can evaluate the situation. In the end, you can create a hard copy checklist for your child to refer to when they feel anxious.