Hi dudeitsmattk,
At least it’s good to see that at least your girlfriend loves the mattress even if you aren’t doing as well with it as she is :unsure:
There is more information about the most common symptoms that people may experience on a mattress and the most likely reasons for them in post #4 here and the other posts it links to. While the most common reason for lower back issues is a mattress that is too soft … your mattress only includes 3" of memory foam (as far as I know because I don’t know the exact specs) which would generally be a firmer memory foam mattress and increase the likelihood that it’s too firm so it’s very possible that in your case the back issues could be coming from a mattress that is too firm.
There are also some suggestions in post #2 here that may be helpful (particularly if you are using a thicker mattress protector or mattress pad that may be affecting the ability of the memory foam in the mattress to contour to your body shape).
If your mattress is too soft it would be much more difficult to “fix” without removing and replacing the upper layers in the mattress which are too thick and/or soft (see post #2 here) but with a mattress that is too firm then adding a topper can certainly help provide the additional pressure relief and/or secondary support (the type of support that “fills in” the gaps in your sleeping profile) that you may need.
If you are confident that the mattress is too firm then post #2 here and the topper guidelines it links to can help you use your sleeping experience on the mattress as a reference point and guideline for choosing the type, thickness, and firmness for a topper that has the best chance of success. It also includes a link to a list of some of the better online sources for toppers that I’m aware of as well.
If you aren’t certain whether your mattress is too soft or too firm then I would make sure that any topper you purchase has a good return policy so you can try it on your mattress with little risk just in case it doesn’t turn out as well as you hoped for because the only way to know with any certainty whether any mattress/topper combination will be a good match for you in terms of PPP will be based on your actual sleeping experience.
Phoenix