I'm sorry to hear about your bad experience. The first thing is to make sure your pain isn't being caused by something other than the mattress. Severe pain should be taken seriously: consider getting things checked out by a physician.
The second thing is to ensure your mattress is supported correctly. Check that the box-spring is oriented correctly and that there are no obvious sags in the mattress. If these things check out, read on...
My understanding is that the Sealy Optimum Cava has the plush pillowtop, then Gel memory foam/memory foam over the "opticore" beneath. It is rated as a "plush-firm" mattress, meaning that the pillowtop will be soft, but the foam will be more firm.
So, will the mattress soften up over time? Maybe, but just a little.
A high-quality plush pillowtop will change little over the short-to-medium term, but long term they are known to develop impressions, simply because the materials inside compact. Lower quality pillowtops don't last as long. The Sealy Optimum Cava is made from reasonably high-quality materials, so you can expect the pillow-top to continue providing a soft surface for several years at least.
Memory foam is a more complicated: if it is high-density (high-quality) then its firmness at a steady temperature will soften up only slightly over its lifetime (Lower density foams soften-up much more but often form impressions). The Cava has relatively high-quality foams, so it will not change much.
The biggest factor affecting memory foam firmness is temperature: warmer=softer, colder=firmer. If the showroom temperature was slightly warmer than your bedroom, then the mattress would have felt softer. You can experiment with firmness by changing the temperature in your bedroom and seeing if you notice a difference.
I hope this sheds some light on the situation.
This message was modified May 17, 2013 by a moderator