This may sound a little weird but try this. I was a chronic mouth breather, and relearning to breath at night through my nose was difficult but here's what finally worked. Touch your tongue to the roof of your mouth and then without moving it swallow. This creates a very small vacuum or rather a suction within your mouth. Try to relax with this situation in place, breathing through your nose. It worked for me.

Brian, stage 4 oral cancer survivor. OCF Founder and Director. The first responsibility of a leader is to define reality. The last is to say thank you. In between, the leader is a servant.