Whistling in the Dark

So, I'm lying waiting for sleep, CPAP mask on. There's just one problem. I'm not lying in bed.

See, if I sleep in my bed for more than five hours, I get a terrible backache. If I sleep on my couch, I don't. So I've taken to sleeping on my couch. I only discovered this recently, as before that the sleep apnea meant I wasn't sleeping that long in bed to begin with.

This wouldn't bother me (much), given my weight problem. I mean, life can be like that. Only... I got a Select Comfort bed, that inflates or deflates at my whim, specifically to ensure I could sleep comfortably without these problems.

I guess part of the problem is I don't know if soft or firm is supposed to be better for heavy folks who sleep on their sides. So it's hit or miss, trial and error, and so far nothing's working quite right. Except, of course, sleeping on the couch.

Advice would be cheerfully accepted.