I found that more often than not, instead of trying to impose rhythm on the breath, observing with genuine curiosity whatever the current rhythm is (deep or shallow? short or long?) and focusing on the feeling of the breath itself as well as assorted bodily sensations such as the chest raising and falling does wonder to calm down. A gentle letting-go of resistance fares better on the long run towards acceptance and relief than downright coercion, which can have a nasty swing-back boomerang effect.