Support dissolves after a couple of days

It had been a ridiculously many years since I bought a new mattress and I was so excited to finally ditch my decrepit bed and get something brand-new and with much newer support technology. Especially since I've been having back pain for as long as I can remember now, and I figured that a new mattress would probably help! The first night was wonderful, and my back did seem to feel better! LITERALLY after the first day--or two at most--I started noticing my body impression. Which seemed odd. Well, it got worse from there. The support erodes after mere DAYS. This thing is supposed to last for a decade? HOW? The warranty says that they'll replace if it sags two inches or more. Well that's not what's happening, and the pillow top fluffs up (i.e. if you lay a yard stick across it, you don’t see much dip), but the problem is that there's no support once you put any weight there! It is a disaster and I've wasted hundreds of dollars on this thing.