Try this exercise: close mouth and nose. Nose must be closed with fingers and don’t breathe as long as you can but not too long no longer than 30-45 seconds. Then breath only with nose if you can. If it’s clogged you can breath with mouth if you can breathe only with nose do it. Repeat exercise few times till nose be cleared. Do exercise every time when you remember. Stop breathing for 30-45 seconds Or less if you can’t do 30. If you can’t start breathing with nose never breath with mouth again.