Which mattress for bulging discs and arthritis in the hips?

I'm 78, female, 84 kilo and 5feet 5. I usually sleep on my left side. I have bulging discs, arthritic hip and now aching top arms and shoulders from sleeping awkwardly. I've had two Tempur mattresses--an"Oslo" from Bensons, which has just been sent back and an original from Dreams. Both have crippled me, although they seemed fine in the shop and I spent a long time trying. I like Dunlopillo but a Royal Sovereign seemed soft. I am returning to a single bed as I think it would be better for me. I have a John Lewis sprung divan base. Whatever I try seems to be fine but far too hard for me when I get it home. I'm thinking of medium but am totally at a loss as to which mattress to buy. Would appreciate your advice,