The cold air is partially why I use a tatami mat under my futon. It keeps it off the ground enough to insulate.
There's good options out there for this style of sleeping. Just keep experimenting until you find something you like. I'm also a fan of folding mattresses. A little bit more soft then a futon but still good you imo.
No worries fren. If it doesn't work just go back to your old routine.
I'm some what well read on this stuff because I've struggled with sleep problems through out my life. My job is very physically demanding so good sleep is important for me.
It's possible on true flat flooring by curling into a slight fetal position. Your weight distribution is more even that way. Agreed that it's not the most practical way to sleep on the ground.
My futon with a tatami mat allows me to side sleep pretty comfortably. Enough softness to alleviate pressure points but firm enough to keep me from bunching up my shoulders.
The futon I use helped a lot of issues caused by my side sleeping. By sleeping on a more firm surface your hips and shoulders maintain better pressure points. The flatter surface allows the body to adjust and move during sleep.
The problem with soft mattresses is they are too comfortable for your body. They allow you to sleep in a way that discourages your body from moving around at night. Creating pressure points that become overuse injuries.
Side sleeping is particularly prone to this. On a soft mattress your hips sink in so your weight bearing shoulder digs into the mattress. This causes the weight of your body to get unevenly distributed. Overnight the ligaments in the rear ribcage get stretched. I wouldn't advise to just sleep on the floor but our bodies weren't meant for soft mattresses imo.