Memory foam mattress killing my back!!!!

I have a "high quality" memory foam mattress similar to tempuredic, I can't remember the brand, its either serta or sealy. I bought it back in 2012 and its slowly gotten soft where my fiance and I sleep. Its a king size and in the middle its nice and supportive where no one sleeps but where we sleep its terrible. I have chronic back pain along with fibromyalgia (I'm a 30 year old female) and this bed leaves me feeling like a block of cement in the morning.
I need to do something. I went and tried out the sleep number beds and the one we liked was going to cost around $4100, and that was on sale, without a base!!
Does anyone have any advice as to what we can do with the mattress we have, or is there another mattress out there that is good for the problems I have?
q memory foam mattress killing my back, bedroom ideas
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