Hi sdmat,
I’m not clear about the specific pain you are experiencing or where in your back it’s located or whether your mattress is too soft or too firm (there are probably more people that would say the Tuft & Needle is too firm than would say it’s too soft) but if there are no signs of improvement after about 3 weeks then I would agree that it’s probably time to return/donate the mattress for something different that is more suitable for you in terms of PPP (Posture and alignment, Pressure relief, and your Personal preferences).
Before you return your mattress though if you believe that your mattress is too firm then there are some suggestions in post #2 here (in particular checking whether the type of mattress protector or any mattress pad you may be using is affecting the feel and performance of your mattress) that may be helpful and if your mattress is too soft then there are some suggestions in post #4 here (in particular making sure your foundation is providing suitable support by testing the mattress on the floor to see if it makes any difference) that may be helpful and would be worth trying before you return the mattress.
I would always keep in mind that the first “rule” of mattress shopping is to remember 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 that are unique to each person 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 or which type of mattress would be the best “match” for you in terms of “comfort” or PPP or how a mattress will “feel” to you or compare to another mattress based on specs (either yours or a mattress) or “theory at a distance” that can possibly be more accurate than your own careful testing (hopefully using the testing guidelines in step 4 of the tutorial) or your own personal sleeping experience (see mattress firmness/comfort levels in post #2 here).
As long as the materials in a mattress you are considering are durable enough for your body type and meet the guidelines here … the choice between different types and combinations of materials and components or different types of mattresses (see this article) are more of a preference and a budget choice than a “better/worse” choice.
In other words … one type of mattress isn’t inherently better or worse than another as long as a mattress you purchase is a good match for you in terms of PPP, uses good quality and durable materials, and compares well to the other mattresses you are considering based on all the parts of your personal value equation that are most important to you.
Phoenix