Problems with my lower back.

Hello there,I was considering buying one of your mattresses and was just wanting your advice on the mattress you feel would be suitable. I was wanting to buy a kingsize mattress for my Tempur ottoman base unit. I'm slim 5'5 and weigh 9st 4lbs (aiming to lose another 1/2 stone). My partner is 5'9 and weighs 13.5 st.I've had problems with my back over the years. I currently have a Tempur Firm Original Sublime mattress. For the first 4 months I loved this mattress but since then it's been getting softer and softer and no longer supports my lower back. I wake up with lower back pain and stiffness. When I'm lying on my back I feel that my bottom is sinking too far into the mattress and that my lower back is no longer properly aligned or supported as the mattress is too soft. I've also found in the past, that mattresses that are too firm for me also give me pain and stiffness in my lower back in the morning (when I lie on my back).Also as I'm slim I need some kind of comfort layer when I'm lying on my side as I don't have a lot of padding on my hips. I find that if a mattress is too firm for me it digs into my hips when I'm lying on my side, and I wake up with discomfort and stiffness in my hip area.I need a mattress that's not too firm or too soft, that supports my lower back properly. I sleep on my back and my side equally.Many thanks for your help,Serena