Possibly look into "mewing" i.e. orthotropics which deals with proper oral posture, breathing, and swallowing. You could also try just putting your tongue on t

Possibly look into "mewing" i.e. orthotropics which deals with proper oral posture, breathing, and swallowing. You could also try just putting your tongue on the roof of your mouth when your mouth is closed (which is what you should be doing anyways). Your tongue is actually what is supposed to help get your jaw to stay up with a suction hold against the roof of your mouth, and if you don't have your tongue in that position then you will have to use only your jaw muscles which can cause to much pressure.