did i purchase a used mattress?

this mattress was awesome for about a year.. then it started sinking. i have to flip flop all over the bed in order to find a spot that i can sleep with my butt or back not sunkin into a groove. i called the store when this started happening and was told it was under warranty, however, any stains on the mattress will void out this.... well, i am a woman over 40.. i get nite sweats so the is sweat stains where i lay... also was told i had to pay an inspector to come out..... i did my best to deal with the mattress as is.. periodically turning it around.... March of this year is 3 years since the purchase and i have not been able to sleep in the bed at all... i weigh about 124 lbs and in three years i have slept in this bed alone.... please tell me how when i get up in the morning it looks like a 300lb person just layed there all nite????? i am beyond frustrated as the reviews for this mattress and this company are unacceptable. it is clearly concerning that i cannot have this bed warrantied due to sweat stains as it causes wonder as to how this is relevant unless you are planning to resell a defective mattress. my mattress seems to have already been slept in for quite sometime in order to create such a deep groove in the center.. DID I PAY ALMOST 600$ FOR A USED MATTRESS?????