Hi Tanzanite,
It may be worth asking the manufacturer of your mattress since it’s unlikely that they are all the same density (although it’s possible).
The possibility that your support system could be contributing to the issues you are having was the reason that I suggested trying your mattress on the floor in my earlier reply here. That would be the best way to decide whether a firm, flat, and non flexing support system (similar to the floor) under your mattress would be a better choice.
Phoenix