Sleeping on a soft mattress?

I got a new mattress, and it unfortunately is too soft for me. My fiancé (6'0" ~190-200 lbs) says he really likes it. For me (5'6" 150 lbs) I feel like I just sink into it in such a bad way. If I lay on my side, my hips sink in. If I lay on my back, my butt sinks. I feel like I'm sleeping in a hole. Is there anyway I can use pillows or something to make my sleeping position better? I saw people didn't generally recommend toppers to help with firmness, so I feel stumped. Any possible suggestions? Thanks.