Kingsdown Malbec what do I replace it with after only 3 weeks of discomfort?

I replaced a Sleep To Live series 200 because it was causing serious problems (back muscles, shoulder tendons, even knee pain!) I had terrible back muscle pain, shoulder pain, popping hip!, all sorts of things I finally realized my 12 year old mattress needed to go.

I purchased a Kingsdown Malbec for almost 4K a fantastic amount of money! I've tried to adjust to it. At first, I couldn't sit in bed without it causing lower spine pain, that has stopped. I've had this mattress for about 3 weeks, plus (not a full month).

In the past week, I thought it was going ok but now I wake up about 4AM every single morning with middle of the back muscle pain. My body (I weigh 120 pounds) isn't somehow being allowed to get even support and that portion of my back (I wake up on my back, I don't start that way) is taking all the strain. Today my back muscles were fine (I took a sleep aid, didn't toss and turn) but my RIBS hurt and it's obvious pressure on my spine is causing a problem for me this morning.

I have more than two months left to exchange/return this mattress. I don't know WHAT TO DO. I need suggestions from someone who is over 65, can't use a firm mattress. Back-Pain Back-Sleepers Back-Support Scoliosis