Stearns & Foster review: poor product---pit

1st of all I researched your matress and decided it was the very best I could buy and so I did. At 70 years old, it would be the last one I planned to have. My other was over 25 years and in good shape, looked like new.
THE 1st matress I purchased nearly $2, 000 marked down from $3, 600 clearly showed dirt on it even though it was wrapped in plastic from the company. it was exchanged and the 2nd one I had to have someone come out to check it out as it seemed to have a pit in the center. They said no or not enough to replace or worry about. I am a single lady, on the heavy side and always seem to end up in the PIT or center. upon rising it is like I have to climb out of the pit to get to the edge and get out. I have rotated it every month as directed but the PIT is always in the center. This position gives my body a bow like effect, thus putting pressure on my knees. their pain often wakes me up, very painful position for my knees. Recently I discovered if I fill in the pit with some folded towels it raises my butt or hips up to a level, level and I am able to sleep much better. However what a shame with having to do this fill-in on a $3, 600 matress and I certainly can't afford to throw it our or buy a new one, actually I blew my savings on this mattress.
I hope you can help me with this problem. I am very disappointed. I expectly await a response from you soon.
Sincerely, Emilyann Emond