Complete sinkhole

We bought this in a king in June of 2017 from mattress firm. My partner and I are 30 and 26, both healthy. It was fine for the first few days, then we noticed it had started to sink under each of us. We propped it up with pillows underneath and rotated it, but it did not help for more than a couple of nights. Now, we are in constant pain. I have horrible neck and shoulder pain every day and have to drag myself to get up in the morning. Headaches are constant. It sags so much where we sleep that it's like sleeping in a hole. Moving to the "hump" in the middle is the best relief. Neither one of us have been getting restful sleep the entire time we have owned it, except when we sleep in a different bed while traveling. We're tired, cranky, sore, and our joints crack and pop all day. It has aged our bodies significantly. We get rid of it in two days for a nice hybrid we found from our local retailer and I absolutely cannot wait to watch it get hauled away. I'll be attending physical therapy if my back and neck don't significantly improve within two weeks. This things is a nightmare, do NOT buy it.