Turned to "mush"

This mattress was great...at first. Now, less than 2 years later after paying nearly $2,000 the feel of it is terrible. It is very mushy and not at all like it used to be. My daughter has fibromyalgia so she likes a softer mattress, but now she is soooo uncomfortable as it sinks like sleeping on a wet piece of foam. We use a water proof, expensive mattress cover and no damage was done to it on our part. She weighs about 100 pounds so I doubt it was from some excess weight! Sleepy's no longer carrys this brand. I'm heading there now to complain....