Too soft, not supportive enough

I'm on my second mattress from Nest. I requested to return the first one and they said they had a new version of the bed that they felt would fit my needs better, mainly that it wasn't supporting my middle section enough and causing my back pain. Well the one came and it's slightly better, but still not supportive enough for me. So, before the 100 days are up I'm going to request a refund. I just don't know what to get yet. In their defense, my wife really likes the bed. It's a medium soft bed and we're both side sleepers so not sure what the difference is, but it's painful to sleep in for me. For now I've been sleeping on the guest bed in the basement until we figure out what to do.