"How exactly can one control thier posture while asleep? "

I have worked on getting to sleep in a posture which I know is safe...usually on my side....preferably right side. I use 2-3 extra pillows to cushion my arms and/or head in various positions. I sometimes wake at night, and use the opportunity to readjust things.
Now & then I DO manage to sleep for awhile in an awkward position, but one of the major issues is how long the pressure on my neck is 'wrong'....the longer the area around the nerve is turned wrong, the more inflammation. A long car drive can give me some irritation that can take a couple days to recover from, but I haven't had really bad pains in a couple of years.

I'm fortunate that I WAS able to get a specific diagnosis so that I can cope.....most of the time.
(here I sit, slouched in my desk chair, looking lazy and with bad posture...but it works..)