Hi Millenor,
You are certainly not alone in experiencing symptoms that are connected to the initial break in period or in getting used to a new sleeping surface.
The break in period for a mattress depends on the type of foam used and on the other components such as the cover materials and quilting as well. There is also an initial adjustment period for any new mattress and sleeping surface as your body “unlearns” it’s sleeping memory from the previous mattress and gets used to the new one. Both of these are usually 90 days or less but in most cases it’s less than 30.
How a mattress feels can also be affected by other factors as well (including the type of mattress protector you use along with any mattress pads or other bedding, the foundation you are using, your pillow, and in the case of memory foam the temperature of the room as well.
There is more about the break in and adjustment period for a new memory foam mattress in post #2 here but once you’ve checked for the other factors that may be affecting your mattress then the first suggestion I would make is to talk to the retailer or manufacturer (in this case Select Foam) because they may have some suggestions that are specific to their mattresses that can help.
Phoenix