You don't want to push up into the nasal space.  We need to get this message to Dr. Mike Mew.  Dr. Singh knows this.  We need to realize that the airway is through the nose/over the maxilla, through the mouth and down the back of the jaws.  The problem is that no credence is given by Dr. Mike Mew on the nasal area.  This is very troublesome and highly problematic for anxiety!  I don't expect that you'd know this...but if you push your maxilla up then it can encroach into your nasal space.  You have to try to get that back down if it went up.  The reason why mouth breathing happens is because of a constricted nasal space.  That's why allergy sufferers mouth breath.   I wish Dr. Mew would realize how bad his ideas are.  Short-term mouth breathing shouldn't be a problem.  Be sure to do yoga and deep breathing (through nose) exercises before bed and avoid a plastered tongue posture on the roof of your mouth.  Speak normal sentences to see where your tongue was.  It often will end up where it was before if you can just do a little speaking then stopping.  Many on here forget where their tongue was to begin and often it was fine.  Unless you looked like Dr. John Mew it was fine. 

How old are you?

Honestly, you can even put your tongue on your lower jaw to be safe... the problem is that Mewing can be highly stressful and it really isn't very good, TBH.

not much a friend of my who has never needed braces and who is pretty well developed breaths through his mouth during sleep and when doing sports like soccer