You have to do what is called diaphramic breathing. Your diaphram is just below your lungs. Thats where what should rise the most while you are breathing, not your chest. You should inhale slowly through your nose and exhale through your mouth.
Walking fast can normalize your breathing since you will concentrate less on it and since anxiety causes over breathing(co2 levels in the blood get too low) it will raise your co2 levels in your blood and make you feel more normal.