You will have to force rhythmic breathing a little by exhaling through your mouth or nose once the breath is complete. It is important that you exhale while your face is underwater such that as you turn to breathe, the lung will have a large capacity to accommodate fresh air. This mistake makes carbon dioxide accumulate in the lungs. It is wrong to hold your breath while your face is still underwater, and then try to breathe quickly and shallowly.
The key to rhythmic breathing is to exhale only after inhaling.