I had to learn to sleep on my back due to lower back pain. It was really hard for me to get to sleep this way. I could nap straight out like I was dead, but not at night. So here is the things that I tried. I prop up my knees with a pillow. This takes some of the pressure off your back. Try those really foamy, never gets dent type of pillow for below your knees. And make sure that the blankets are not pulling down on your feet. I also had to give up a pillow under my head so that I was totally flat from hip to head. That was really traumatic, I had something like 8 pillows. I would put a pillow along side my face and sorta tip into it. I hope this helps you. It will just take time to get used to the new position. By the way, why do you need to sleep flat?