Wake up feeling like I slept in a hammock

This mattress was quite comfortable for the first 1-2 months that I had it. My husband says he still likes it but I definitely have strong buyers regret. In the short time that I’ve owned it I just can’t believe how much it has CHANGED. First of all, it doesn’t bounce back to flat after I change positions, EVEN after I get up to use the bathroom. It gets a large and deep depression where my lower back and hips rest, which makes it quite difficult to get comfortable when I lie back down on it. But worst of all I wake up with a really sore lower back. I am not exaggerating when I say that I feel as if I’ve been sleeping in a bad hammock. I’m incredibly disappointed, and frustrated by the memory I have of the Beautyrest mattresses I had back in the days before memory foam, when a great mattress could be purchased for far less than $1,000.