Major Body Impressions

We have had this mattress for 2 1/2 years and it's awful. The first 3 months were ok and then it quickly started sinking in where we sleep. Now, after 2 1/2 years we have large cavities or body impressions on each side of the mattress. We have a king. When you get on the very outer edge you roll into the cavity and same with the very center of the mattress. We have flipped it around, but since it's a one sided no flip mattress we cannot flip it over. We are not heavy people either. I am 120 lbs and my husband is 170lbs. This is our 3rd mattress in 3 years. We started out with an expensive air mattress - select comfort. Then we tried a Beautyrest which also caved in after 6 months. We sent that one back and got this Sealy one. We are now trying mattress padding - memory foam and other, but nothing is working as the bed already has the cavities so anything on top is just sinking into them. We will likely try a new one soon. We thought both Beautyrest and Sealy made good mattresses. Maybe they used to. My grandmother swore by her Sealy, but apparently they have changed.