Firm pillow top feels like feather pillows

I have a bad back and bought a firm luxury pillowtop with the urging of the Macy’s employee. I should have known better. I previously had a Serta posturpedic for 10yrs and was moving and thought I should get a new mattress even though nothing was wrong with it. I’ve had this mattress 2yrs now and 8mos ago i started waking up with back, neck and shoulder pain. My mother came to visit, slept in the bed and said it was time for a new mattress because it was sunken. The luxury firm pillowtop feels like I’m sleeping on a feather bed/pillow and sinks in like a hammock. I can’t wait to get rid of this and hope Macy’s will let me switch for another manufacturer