But, if you can breath through your nose well enough, I think some kind of mouth tape can work? I haven't tried the Hostage brand anti-snore tape, but maybe someone here has had experience with that.
If you really want the chinstrap that I got, I got this one: https://www.amazon.com/gp/product/B08DR16XGV/
Based on the pictures, it obviously pushes the jaw backwards. That obviously wouldn't work for you. I wear mine in such a way that it is helping my jaw close and stay shut. I agree that you don't want something pulling your lower jaw backwards.
Though I have this paranoia that mouth tape will increase my philtrum length, which I know is a vain worry. Another solution is one of those incline pillows - just a 20% gradient should allow me to back sleep, which I prefer.
Post-surgery I had my jaws wired shut and they had this chin strap that was like an ace bandage that around my head and chin. I could replicate that with one of those mouth guard combined with a chin strap, but I read somewhere that this would eventually pull the upper jaw down towards the airway as well (since maxillary plates are not fixed.)
Paid for a sleep study and the doctor walked in the room and said, "You don't have sleep apnea." I was like, " I knew that, but what can I do?" The doctor shrugged and said, "You have hypopnea. You could try a mouthguard...?" trailed off and left.
So, with this newfound wisdom I was back at square one.
Some things that dramatically helped me in recovering from years of sleep loss: - A chinstrap. - I found a study (n=1, literally) about a patient who didn't fit the typical sleep apnea profile as a thin older man. The doctors treating him wondered if it was "positional sleep apnea" and had him get a chinstrap. This held his mouth shut while sleeping and prevented his tongue from falling back and blocking his airway. I found a chinstrap on Amazon and immediately started sleeping better.
- A blood test - My doctor knew that I was dealing with adrenal issues due to not sleeping, Lyme disease, and other issues, so he ordered a blood test. This was fantastic. For me, addressing my underlying magnesium and B-12 deficiency made an enormous difference in sleep quality. I found that taking my B-12 right before bed helped my sleep be so much more restorative.
- Andrew Huberman - Andrew Huberman's podcast Huberman Labs is old news, now, but when I found him a few years ago, it was some of the more actionable advice I had ever gotten about sleep, falling asleep, and circadian rhythm.
Anyway, there were more things across the way, but these are some things I shared with others that they also found helpful. I now am in control of my sleep and feel decades younger. You can get there! It's worth experimenting and finding a doctor who is going to help you find your problem. The mental clarity, focus, and energy has definitely made up for all the the months of trying to find out what was wrong with me.
I hope you sleep well!