There's a reason why doctors don't want you to sleep directly on your back. There's a main artery that can get restricted from the weight of the baby and uterus falling towards your back.
U should be fine in your back. I think they say left side is the best cuz it helps the blood flow and helps u to breathe better. Idk how u sleep on your back... I can never breathe like that Lmao. It's hard for me since I've always slept on my belly.
While you should lay on your side as much as possible, your body won't let you hurt the baby by sleeping in a position. You're fine.
I say sleep whichever way you can while you can. Im 35 weeks and can never sleep
You can sleep on your right side too. It's perfectly safe. Doesn't always have to be the left side. If you find you're sleeping on your back you should prop one side of yourself up with pillows so you're not fully laying on your back.