But breathing is such an automatic habit. How can I change it?

Again - Breathing is the only involuntary bodily function that can be consciously controlled.
What does this mean? Even though we do it all the time, we are not aware of the power and effect it can have. We cannot consciously take control of our digestion system (other than the initial chewing and mastication process), our blood pressure, our endocrine systems, etc., but we can take control of our breathing patterns. And, mysterious as it may be, through the breathing we can affect all of those other systems. Relaxation techniques are all derivatives of breathing. Digestion, blood pressure, and improved heart rhythms are all examples of improved health without drugs, brought about by using correct breathing techniques.