Good quality. Not right for me, ok for partner

I don't sleep as soundly as our old mattress. I've been sore or stiff every morning for over a year now, in the back, neck, hips, and /or shoulder. I can't sleep on my front because it hurts my lower back. Can't sleep on my back because it leaves me stiff. This mattress contributed to shoulder impingement on my right side, so that's out. I can only sleep on my left side, and must thoroughly stretch my whole body when I get up. I never slept like this on other mattresses that weren't memory foam. My partner sleeps ok, but they're not as sensitive as I am. We're way past the 100 night guarantee, and it's too expensive to replace.