Very stiff back and hips

My current bed has a memory foam top layer, has 1000 pocket springs, and is on a divan base. It is over all a very thick and heavy beast. I wake up several times during the night because I find it hard, and it sags in the middle even tho it is only 3 yrs old. I am (unfortunately) 12 st 12 lbs, 5 ft 6 ins. I wake up feeling very stiff in my back and hips I am not really comfortable on my sides or back. I feel that maybe I need more support, but softer for my hips. Thank you for your advice.