Hi kefit,
I don’t think your mattress has “failed” and your experience sounds more like you made a firmness choice that was “on the edge” of being too soft for you and that the relatively small amount of foam softening that can happen in the first months of sleeping on a mattress that is normal for any mattress or material was enough to put you “over the edge” and outside the firmness range that is suitable for you (see post #2 here).
Sometimes when the comfort layers are a little softer than your ideal then you can “go through” the comfort layers and feel more of the firmness of the deeper layers underneath them than you are comfortable with so as paradoxical as it may seem … sometimes a comfort layer that is too soft can lead to a mattress that “feels” too firm.
In other words … this seems to be an issue with the suitability of your firmness choice not an issue with the durability of the mattress or the mattress “failing”.
If the pressure you are feeling is coming from the deeper layers then a 1" or 2" topper could certainly add to the pressure relief and comfort of the mattress and could be an effective solution. The key is to choose the thickness and firmness that would be “just enough” to isolate you from the firmness of the layers you are feeling but no more so that there is less risk for alignment. If you do decide to go in this direction then post #2 here and the topper guidelines it links to can help you use your sleeping experience as a reference point to choose the thickness and firmness that would have the best chance of success.
While this is certainly a high quality and good value mattress, if I was in your shoes I would probably consider trying a topper first since your mattress would still be in good condition (the latex comfort layers in your mattress are a very durable material). The odds are good that you only need some fine tuning and I would only tend to consider a new mattress if a topper turns out not to be an effective solution for you.
Phoenix