Deep breathing will activate the vagus nerve and calm you down in turn.
Pause your breath on the exhale to a comfortable extent, and let the exhale be relaxed and not forced at all.
Also lie on your back and put books on your diaphragm to see if you move them when you breathe or not. If not you can practice by tying a belt around your upper ribcage and breathe like that to learn how to inhale with the diaphragm.