@cjj2 another thing you could try is bed risers (they come in a pack of four, it you need only two) under the two legs/casters at the head of your bed.

I used bed risers for many years, but recently bought an adjustable bed which is great because I can raise or lower the top of my bed as needed with a remote any night. Some nights I need my bed raised higher than on other nights.

@cjj2 I use a wedge pillow and sometimes even a pillow under my knees. I still slide down through the night but not too far. I used to sleep all curled up in a fetal fasion until the need to sleep on my back became important. The wedge helps a lot, but I also stay up later so I'll sleep quicker and when I finally lay down on my back I do meditation and concentrate on feeling the whole of my back including shoulders touching the pillow. I ordered this: "Bedluxe Wedge Pillow Headboard, 3PCS Orthopedic Bed Wedge Pillows Set for After Surgery, Adjustable Pillow Wedge for Acid Reflux&Back Pain Relief, Sleeping, Back Pillow for Bed Sitting Up" from Amazon.