Hi BackHurts,
It’s true that all new mattresses will have a break in and adjustment period where the materials will soften a little and your body will get used to a new sleeping surface that is probably different from what it’s used to (see post #3 here) so it’s usually best to wait for at least a few weeks before exchanging a mattress or making any other changes if that’s possible.
If it’s just not possible to wait for a few weeks because because you are in too much pain or discomfort then of course you would probably need to decide on what to do more quickly.
There are also two versions of the Myrbacka mattress and one is a medium firm mattress that has latex comfort layers and a polyfoam support core and the other is a firm memory foam mattress that has memory foam comfort layers and a polyfoam support core so it’s possible that you tested one but ended up either ordering or receiving the other one. You can check the materials listed on the law tag to see which one you have and compare it to the law tag on the mattress that you tested in Ikea.
The biggest reason that most people end up choosing a mattress that isn’t suitable for them to sleep on or that causes them pain or discomfort is that they only test a mattress for a few minutes in a store and then only test for “comfort” or the “showroom feel” of the mattress which often doesn’t translate very well into how they will actually sleep on the mattress in “real life”. There is more about testing a mattress for PPP (Posture and alignment, Pressure relief, and Personal preferences) in the links in step 4 of the mattress shopping tutorial.
Both of the Myrbacka mattresses are foam mattresses … they just contain different types of foam. One contains latex foam and polyfoam which would tend to sleep cooler and the other contains memory foam and polyfoam which would tend to sleep a little warmer (in very general terms memory foam sleeps warmer than latex).
It certainly sounds like the mattress you chose is too firm for you which means that you could either exchange your mattress for a softer Ikea mattress or you could add a softer topper to provide the additional softness and pressure relief that you need.
I would keep in mind that you are the only one that can feel what you feel on a mattress and there are too many unknowns, variables, and personal preferences involved to use a formula or for anyone to be able to predict or make a specific suggestion or recommendation about which mattress or combination of materials and components would be the best “match” for you in terms of PPP (Posture and alignment, Pressure relief, and Personal preferences) based on specs (either yours or a mattress) or “theory at a distance” that can possibly be more accurate than your own careful testing or personal experience … hopefully using the testing guidelines in the tutorial post (see mattress firmness/comfort levels in post #2 here).
If you decide to exchange your mattress for a different Ikea mattress I would make sure that you test the other Ikea mattresses very carefully for PPP because you only have one exchange.
The other option you have for a mattress that is too firm is to add a topper to provide the additional softness and pressure relief that you would need. If you do decide to go in this direction (either with the mattress you have now or with a mattress you exchange for if it also turns out to be too firm) then post #2 here and the topper guidelines it links to can help you use your sleeping experience as a guideline and reference point to help you choose the type, thickness, and firmness for a topper that will have the least risk and the best possible chance of success.
Phoenix