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How to stop mouth breathing
Try to breathe through your nose for 2 minutes.
Close your mouth, watch a clock, and try to breathe through your nose for 2 minutes straight. If you have issues doing this, it probably means that you have a clogged nose and the cause of your mouth breathing is physical or structural rather than habitual.
If your mouth breathing is caused by a structural or physical issue, you'll need to investigate further and get diagnosed by a doctor. If you have no issues breathing through your nose, then it's a habit and may be easier to fix.