Sleeping on a Hill

I bought this mattress just under a year ago. It was on sale because they just got the "new line" in so it was only 800.00 before taxes (its normally over 1000.00). I laid on every bed in the store as I need FIRM support. I told the guy I had back issues and needed a bed that was good for back support. He suggested that one and It FELT SO GOOD. Well half a year later the box spring broke and now there is just nothing left to this bed it is FLAT I am talking so flat (except in the middle) that we can't even keep a sheet on it. There are giant craters on all four corners of the bed. its lumpy and there is no back support at all. I am in more pain now than I was before and would much rather sleep on the floor. my bed looks like this ^ that is what I am trying to sleep on! I don't know how you sleep on a mattress that looks like ^. I would not recommend this to anyone. I am actually contacting the store tomorrow to see If i can get a new one under the warranty but I will not accept the same type of mattress!