You could try adding a topper. This can transform the feel of your bed, making a mattress much firmer or much softer. However, even firm toppers will sag if the base they're put on isn't stable – even the best topper can't fix a sagging mattress. Your best bet for improving edge support would be to go for a topper that has more structure to it, such as the Saatva Micro-Coil mattress topper, but temper your expectations.