Marshmello in one place, hard in another place

This has been a very strange experience, and I never thought I would have a problem with a mattress. I was looking for a fairly firm mattress and liked what I experience at the Mattress Firm store. When mine was delivered it felt too firm and I called the store to get it returned and they said I could not have my old matress back and to give it some time. Now that I have done that they say I missed the cutoff. I am ready to give the matress, sheets and everything away. I have to move around to find a spot that is firm and not too soft. I never have had a matress that changed so much from when it was new. Big mistake!!!