question for all pregnant women and mommies!

Okay so my doctor told me that by now I shouldn't be laying on my back because it could press on my air ways and I could have a hard time breathing and it could possibly deprive the baby of its oxygen supply. I'm 19 weeks. But literally the ONLY way I can fall asleep is laying on my back. It's the most comfortable position for me to fall asleep but I feel like I'm hurting my baby so I turn over on my side. I usually end up falling asleep but it's because if I lay there long enough I'll get bored and go to sleep. Did anyone else have or does anyone have this problem?