Quickly became uncomfortable

Purchased this at Sleepies... a model made just for them by Beautyrest. After testing many models in stores, I really loved this mattress when I first purchased it. But in under a year, "gullies" developed. (I am not, by the way, "plus sized"). The mattress sloped down to areas where I slept most, so I began positioning myself on the "higher" ridges to get the back support I was seeking and to even out the wear on the mattress. After 3 years, there are no comfortable areas of the mattress. There is no firm support anywhere. In recent months, for the first time ever, my mid-back hurts when I'm in bed. It's painful when I awake and first get out of bed, with twinges of mid-back pain during the day. While I used to look forward to climbing into bed, I now dread it and have to shop for another mattress. This was an expensive mattress, and definitely not worth what I paid to require replacement in 3 years. Huge disappointment.