Hi boohicks,
The best suggestions I could provide are first to read post #1 here which will give you lots of basic information about mattresses and various materials and types of layering that will help you know and understand why you may be having these issues and some potential solutions.
You are not starting from scratch though and only have the option of an exchange (if they will allow a second one all) so the next suggestion I would have is to read post #66 here and the other threads it links to which are discussions with people who have had to deal with the same issues (an exchange with a retailer who really doesn’t carry any good quality/value mattresses) and suggestions about how to make the best of a bad situation and some suggestions and “best possible” solutions and choices that may be available. Do you have the option of another exchange?
If an exchange is no longer possible … I would give the mattress more time so that the initial break in period along with the adjustment period that is part of any new sleeping surface is complete and “stabilized” and then try to identify as specifically and objectively as possible (in terms of pressure relief and alignment) exactly what you would like to change about your mattress and the specific “symptoms” that you are experiencing.
Post #6 here may be helpful as well.
Phoenix