Unfortunately your experience is not unusual … especially with most of the major brands that use lower quality materials in the comfort layers that can soften or break down much too quickly.
The odds are good that the issues with your mattress are with the comfort layers and not the springs underneath them (a mattress will normally soften and break down from the top down).
Post #4 here has more information and suggestions about mattresses that have developed soft spots or sagging (or were too soft in the first place) that may be helpful.