lying on my back at 3rd tri

Hello, Mom. I'm really confused right now. Entering the third trimester, my body feels completely out of sorts. Pain, aches... the only thing that makes me feel better is lying on my back or at least half-lying down (leaning back). But they say as the baby gets bigger, you shouldn't stay in that position for too long... I can't take it anymore, I cry almost every day because my body feels like it's falling apart. Does anyone have any advice? 😭