Try to screen out any disturbing noise or light. Dim the lights or turn them off. Stretch out on the bed or sofa and roll your shoulders around a few times. Now you can focus on your breathing. When you are tense, breathing can be irregular and forced.
Most people also use their chest to breath when they are upset.
The most natural way to breath is to use the diaphragm which pushes down into the abdomen. Breathing should be relaxed and rhythmic. Breath without using your chest at all. Support your head on a pillow in order to relax your neck muscles. Take several deep breaths into the abdomen.