I had bad sleep apnea until last year, sleeping 8-9 hours a night and usually feeling tired in the morning.

As of the last 12 months it seems to be heald, after I used mouth tape and a nasal dilator consistently for 30 nights in a row. I highly recommend trying this, it was quite cheap (~$20) and the change seems to have been permanent. I used 3m micro-pore tape that doesn't tear the facial hair, and the nasal dilator I used was called "woody knows", though there are many brands that may work just as well. I barely snore anymore where I used to be a raging snorer according to those in the know and my own audio recordings of myself. Most importantly, I only need about 7 to 7.5 hours of sleep now and I will wake up feeling well rested most days. I can also breathe exclusively through my nose now where before I could not. I can take very big breaths through just the nose and do this now when exercising.

I don't think it's all perfect though as I still often breathe through my mouth. I'm starting to wear the mouth tape at night again as an experiment - it really isn't that annoying.

Relatedly, I recently started looking into "mewing" after a breath-work teacher I took classes with recommended it to me as she has been doing it with good results (improved breathing). It seems related to this as Mew recommends keeping the mouth shut all the time. I intend to try the mewing exercises and see if that helps my breathing further.