There are a number of things to try. Put a pillow under your knees. This makes you hips/pelvis more comfortable when on your back.

Practice sleeping in a recliner. An adjustable bed serves this purpose well.

Use a pillow that curves up around your ears. Many need side support. I roll the end of my pillow so it forma an M.

I started trying to sleep on my back decades ago when I developed severe Carpal Tunnel Syndrome. I needed to wear wrist braces at night. It is miserable to sleep on your side with wrist braces. I would sleep on my back with my hands and wrists on my belly.

Most important. Don't go to bed until you are sleepy. If you are truly sleepy, you should be able to fall asleep within minutes. This makes tossing and turning less likely. Being sleepy at bed time starts with when you get up in the morning. No naps. No caffeine after 12 noon.

I finish getting ready for bed/getting sleepy with most of the house lights off. Just a dim light to read or use my laptop. Nothing stimulating to read or watch. When my eyes start to feel heavy. I quietly and calmly go get in bed. No stops for a drink or bathroom. That was done earlier. If it is cool, I use a warming mattress pad so my bed is not cool, but warm and cozy.

When I do this right, I fall asleep quickly and wake up in the morning on my back.

It can require some experimenting to find what works.

Good luck with this. When you find the solution, sleeping and waking up becomes a great experience. Before, I was anxious to go to bed because I would have nightmares and other stressful events during sleep.