One solution is to replace your mattress with a foam mattress. Go to a place that sells them and have the foam unit cut SMALLER than your present one - at least two or three inches in width and length. That gives you the ability to bend the mattress up and back when putting on the bottom fitted sheet, and to just crawl over a bit to push down into the extra space the sides and top of the top sheet and whatever cover you're using. Plus, you can choose the density of the foam for your personal comfort.