I know I need to replace my current mattress, but I can't figure out whether I need to go firmer or softer.

I try to sleep mainly on my back, mainly as these days it's too painful to sleep on my side so back is simply better. When I sleep on my back, I have a lot of hip and lower back pain sleeping as I believe my hips are sinking lower than the rest of my body because it's too soft.

However, when I try laying on my side it feels like there's so much pressure on my arms/shoulders and hips and it's extremely painful, so...that feels like too hard?

I have no idea if I should be looking for a softer mattress or a harder mattress.

Help?