Hi SleeplessinMT,
I can certainly sympathize with you but unfortunately both the initial break in period of a mattress and the adjustment period of sleeping on a new surface is a reality that is part of the process. There is no way to avoid either although the process takes longer for some than for others (for some people it can be right away and for others it can take most of the 90 days).
There are really two issues involved here as well. One is the “accuracy” of your initial testing and how closely you were able to replicate and “predict” your sleeping experience in a store and the other is the break in and adjustment period.
If the mattress really is suitable for your needs and preferences in terms of PPP (Pressure relief, Posture and alignment, and Personal preferences) and it’s only because it’s different from the mattress in the store … then it’s just a matter of time (assuming you received the same mattress that you tested). If the design of the mattress isn’t suitable for you … then the break in and adjustment period won’t “fix” things and you will need to take other steps.
Post #2 here as well as post #2 here can give you some things to check and test and post #6 here has more detail yet and may give you some insights into what may be happening and what you can do to resolve any “symptoms” you may be having.
As LEW mentioned you can speed up the process by walking on your mattress which will have an even bigger effect on memory foam than it will on latex. All of this of course is assuming that the “cause” of the symptoms you are having is in the mattress itself and not something either over or under it or in your sleeping environment (such as temperature) that is changing the way if felt and performed in the store.
Phoenix