Sleeping in a hole

This mattress was great for nearly a year, but I noticed that the area near my butt was beginning to sink about 10 months or so into owning it. At the time, I was the only sleeper, so I began sleeping on the other side. Within a few months, same thing. I rotated the bed, which helped for a short time, but then the same began to happen again. I've had this bed for seven years now, and it's caused me lots of back pain. Some nights I actually have to put a pillow under my hips to keep me from sinking. Other nights, I just get out of bed and move to the guest bed because I cannot stand it. The sinking doesn't bother my husband as much. I can finally afford to buy a replacement. I cannot wait to get a better bed. This one was such a waste of money.