New mattress is too hard for me but is fine for my husband

We bought a new mattress and metal bedstead last year. I find it way too firm but my husband likes it. It's causing me to have back pain and I can't get comfortable. I'm considering buying a new mattress and base but before I do this I was wondering if it is possible to soften my side of the bed. My husband is 16st and I'm 9.5st. We currently have a king size bed. The bedstead has sprung slats and we have a springs/" title="pocket spring">pocket spring mattress. I'm open to any suggestions that you can offer.