Very lightweight at six stone which mattress?

Hi John and Ryan, I hope you can help me choose the right mattress. I have been through four mattresses in the past 18 months!!!

I weigh only 6 stone. Initially I tried a Relyon memory pocket 1200 mattress and didn't find it comfortable due to the sinking feeling of memory foam and waking up with upper back pain every morning. The next mattress was one from a local bed shop which was extremely firm for me. After that I bought a 1500 pocket sprung mattress (with a label name of Deep Sleep) from another small bed store. It was a good mattress, and it was specified as medium firm, however I found it too firm as well. After an uncomfortable time with those mattresses, now I have a soft open coil mattress, bought from Benson Beds, however it is giving me neck, shoulder and upper back pain. All this has been such a waste of money too. I really don't know what to choose next. I would be very grateful if you could suggest a suitable pocket sprung mattress for my body weight. Many thanks, Rita .