D.B.
There are many different masks, and the company that provided the machine should be sending someone to help you make the right choices. That may mean trying on a couple of different masks. I have a CPAP but the masks are the same as for a Bi-PAP. Some of the masks did irritate my skin so it pays to try a couple. I am a side sleeper and the mask shouldn't get in the way - try a different pillow. The pillows that are really firm sometimes press up against the mask and cause them to shift, which makes air leak out. The mask should not be pressing on your nose at all unless you are using the "nasal pillows" type, which I found useless and uncomfortable. If it's pressing on your sinuses, it's too tight, so loosen the straps. The best one I've found is with a very soft and pliable triangular shaped mask attachment - it shifts with my movement. Once you get the right mask, you shouldn't be holding yourself in one position, which is why your back is a mess.
Your doctor should be writing a prescription for the mask of your choice, and setting you up with a respiratory company who will send a rep to your house. I would lie down on the couch or even on top of your bed and roll from side to side as you do at night, and tell the rep what happens to the mask when you do that - which side it's leaking on, where it hurts, and so on. If you're uncomfortable having a stranger in your bedroom, have a friend or family member with you.
Also, the machine may not be set correctly - it should be double-checked at the same home visit.
I also found greater relief through nutritional supplementation which reduced the severity of my apnea, and I sleep much better. I'm working my way off the machine at this point.