Physio tells me breathe in through your nose and out through your mouth. Don't try too hard to get rid of air in lungs, just gently breathe out. Apparently that allows more oxygen to stay in the lungs when you are breathing. That apparently is how to breathe when using oxygen as well...

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Look at https://m.youtube.com/watch?t=868s&v=VR7QnS...

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Try to get into a pulmonary rehab program at your local hospital.