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I know this question may seem counter intuitive, but are you sure you are not engaging too much of your posterior third? It is my understanding, that if you can put your tongue on the “nnnng”-spot(when you say “sing” for example), you are then hitting the right spot. Any further back is not necessary(If someone else here knows if am wrong though, please correct me) This was the case for me anyways. Like you, I couldn’t breathe in the beginning. After watching Astro Sky’s tutorial however, I realised that I was engaging too much of my tongue.
Note that I am on the verge of turning 25, and that I have made progress after 5-7 months of mewing. Hope this helps.
As you undergo correction in the near future, please consider keeping records for your own sake and for others. Pictures of dental impressions, scans, medical reports reports can be very helpful even with all personally identifying information blocked out.
Your input could help many, many people