Comfy but you lose sleep space...

Bought a twin for my young son. Very comfy and easy to set-up. My issue is that the edges of the mattress collapse with pressure. This is my first memory foam bed so maybe that is the case with all such mattresses. But with a twin, it means you lose a large percentage of the actual sleep space. When I lay down with my son I can't go anywhere near the edge because I start to slide off or at best I'm on a slope... Back it goes...

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