There is this guy John Mews, and his son, who advocate something called Orthotropics. It's basically forcing yourself to push your tongue onto the roof of your mouth. I recently got braces, like a few weeks ago, as a 26 year old male adult, and have been doing this. I've been pretty consistent daily, not all day, about keeping my tongue on the roof of my mouth. I really recommend just reading it. It probably is just a placebo, but the cool thing is that I can feel the difference in my breathing. It feels smoother and less forced coming out through the nose. Just the idea of keeping your lips together seemed to have helped. Hopefully over time it improves.

https://www.youtube.com/channel/UC6eh59CVGAMTjxo6q9gCMZA