Firmness fails after just a month or so

I weigh 160 lbs and am 5'9". I have a bulging disk. When I lay on my left side the mattress sinks in by my hip and causes the bulge to be pinched and leaves me with excruciating pain. It is not quite so bad n my right side but it is still painful. The only way I can sleep is on my back. However, when my husband leaves for work I slide over to the middle of the bed where it is still firm (though not as firm as it originally was) and it is not so painful. I had read the reviews on this bed and all raved about it except for a few who said it lost its firmness. And that has certainly been our experience.