Doesn't feel like the one I paid for..Hip is in extreme pain

I used, for years, the Sealy adjustable mattress and very satisfied but adapters broke and Can Not get replacements. Bought this mattress, Sealy Ellington Plush, and sat and laid on it in the store. Very firm but with a bit of softness just like I wanted ! The one that I ended up with sinks about 8 inches when I sit on it, Weigh 155lbs, and when I lay down my body sinks about 3 inches, Too Much, and causes extreme hip and lower back pain that doesn't ease until bedtime and it all starts over. Very disappointed with my $711. purchase. Called and they want $140. for restocking fee. Plus now, 2 weeks after purchasing, they are on sale for a $150 Less.