Hi HeyFaHey,
Welcome to the Mattress Forum! 
I’m sorry your T&N mattress isn’t working out well for you. 
But unfortunately, it’s not possible to “diagnose” mattress comfort issues on a forum with any certainty because they can be very complex and there are too many unique unknowns, variables, and complexities involved that can affect how each person sleeps on a mattress in terms of “comfort”, firmness, and PPP or any “symptoms” they experience. There is more about the most common symptoms that people may experience when they sleep on a mattress and the most likely (although not the only) reasons for them in post #2 here.
You don’t state what type of problem or area of your back where you’re having issues, only that you have “back pain”. Adding the extra plushness on top didn’t seem to help with your issues, so perhaps it’s an overall issue with the type of polyfoam used in this mattress not agreeing with you. Usually when someone is “slight” and sleeps on their side and they are uncomfortable, it tends to be an issue of not enough surface comfort. But in your case you added more comfort and it didn’t improve things for you. One thought I would have would be to flip the mattress over and sleep upon the bottom (mattress “upside down”), which would be the firmer polyfoam layer of the mattress facing upward, and see if you feel better with that level of comfort. The other thing I’d recommend would be to take a look at the foundation you’re using under your mattress, because if that is not a flat and firm surface, you will have issues regardless of how your mattress is configured on top.
With the information you’ve provided, that’s about the best advice I can offer for now.
Phoenix