Mattress Sags in the Middle within 2 months

I have had this mattress for approximately 6 years and I regret every day that I bought it. When I realized it was a dud, I was past the time the store would take it back for exchange. It was as hard as a rock when I first got it. I had exchanged another mattress for this one because the first one got the sag in a few weeks. But within 2 months this "firm" mattress started getting the deep dent in the middle of the bed and has gotten worse in following years. I have the worst hip pain. You can't roll over on this mattress; you have to flip yourself over. The mattress is extremely hot. It is hard to rotate; it's like trying to push a huge marshmellow. Rotating the mattress doesn't change anything, even though the store told me it would help it to "bounce back." I do not recommend this mattress under any cirumstances.