Dont stress on it. You may not be ready to sleep fully on your back.

Focus on lip seal and light mewing. 

Practice Pranayama and Wim Hof breathing before bed. Lay on your back until youre able to relax and thn just go to sleep with your mouth taped if you need it.

As for pillows, anything will do to be honest.

Once youre able to relax your body and breathe deep from your nose youll ditch the pillows and use your arms

@roflcopters I do only sleep on my back, for years now and no issues with it. I was noting how my neck is sore in the morning and it makes sense given the pillows make my head go forward. I also do pranayama before bed and when I wake up. The issue with breathing through my nose seems to be a deviated septum because I wake up congested though am nose breathing all night with tongue on roof of my mouth. I will still try the mouth tape but will likely need nasal dilators to be able to breathe freely through my nose. Before all that though, will do a sleep study to get a baseline.

@vplatt

Seems youre fine if youre sleeping on your back comfortably the whole night.

As for the congestion and the deviated septum i recommend running or any other type of cardio. Developing your cardiovascular system Will boost your nose breathing capacities to a whole new level and get rid of the congestion.

I have a deviated septum aswell and my left nostril clogs easily.

Drink a few beers to relax. It gets you in the mood.

Try a very thin pillow. I got a contour foam pillow at sleepnumber that is thinner than what I see on the website. It's super thin, I just put a pillow case on the contour foam and skip the outside. 

https://www.sleepnumber.com/products/pillow-pillowology

I've got the same problem and only use this for some side sleeping, I can't even use this thin one for back sleeping.

@skinny7 what do you do for back sleeping then? no pillow? when I tried that it was clear from the undereye circles that the blood / lymph wasn't flowing well.

@vplatt Personally I don't use a pillow back sleeping. It pushes my head farther forward. Sometimes I'll wake up in the middle of the night and if I'm too stiff I'll put it in the side, or if I'm lying in bed looking at my phone I'll sometimes use it behind my head. Never when back sleeping. 

@skinny7 I tried that but on my mattress, woke up with pronounced undereye circles (and I did it experimentally, testing and then using a pillow). So it seems the blood / lymph isn't flowing well without a pillow at all. Lately I've been using a flat one that conforms to my neck and it seems ok bu not like nighttime physical therapy which is what I would like.