★★★★★
One of the following may be worth trying. If the mattress is too hard, an upholsterer(furniture maker) would sell you a thin piece of foam that cold be laid on top of the mattress and under the sheet. This foam is used in different thicknesses by manufacturers.
If the bed is too soft - a piece of plywood or any other soft wood laid between the mattress and the bed base can be helpful. This certainly helped my Mom when she had a bad back. After about a year she felt able to remove it when she bought a new different mattress but put it back again a short time later as the new one brought back the old pains.
