I'm not sure if it started this way, but I mostly breathe through my mouth at night due to significant congestion in my nasal passage. If I use a nasal rinse or flonase it helps but if I'm not diligent it regresses immediately.
Give this a shot for a week or two. Just jam these up there. It will be uncomfortable at first but you get used to it. Really opens you up and takes away a ton of resistance in nasal breathing.
I know someone who has to make a "vacuum seal" with their mouth, that is, make some slight suction on the inside so that they won't drool in the night. Is it anything like that?
I had to have my deviated septum surgically corrected back in my late teens before I was capable of breathing consistently through my nose. Even simple things like chewing with my mouth closed were difficult because I couldn't breath well enough with my nose only.