THE SIGNS. A mattress that is too soft for you tends to produce a recognisable set of complaints. You wake with an ache low in the back rather than between the shoulders. Changing position takes effort, because you have to lift yourself out of a hollow before you can roll. Your hips feel lower than your shoulders when you lie on your back. Getting out of bed involves climbing rather than standing. And you feel hotter than you expect, because the more of you the mattress surrounds, the less air moves around you.
SOFT FOR YOU IS NOT THE SAME AS SOFT. Firmness need scales with body weight and with sleeping position. The same mattress can be correctly supportive for a 130 lb side sleeper and far too soft for a 230 lb back sleeper. Side sleepers generally want more give at the shoulder and hip; back and stomach sleepers generally want less, because the midsection sinking is what puts the lower back into an arch. If your partner is comfortable and you are not, the mattress is not defective — it is a firmness mismatch.
IS IT TOO SOFT, OR HAS IT SOFTENED? This distinction decides what you do next. A mattress that was wrong from week one is a firmness mismatch. A mattress that felt right for two years and now feels like a hammock has changed, and that is either normal comfort-layer softening or the beginning of support-core failure. Strip it bare, leave it uncovered for a few hours, then measure across the hollow with a straight edge and no weight on it. A measurable hollow means you have a durability problem, not a preference problem, and it may be a warranty claim.
WHAT ACTUALLY FIRMS UP A MATTRESS. Honestly, less than the internet suggests. A firm topper — latex or high-density foam — placed over a soft-but-sound mattress genuinely changes the surface feel and is the only add-on that reliably works. A cooler room firms up memory foam, which is temperature-sensitive, so a bed that feels like quicksand in August can feel noticeably more supportive in a cooler room. A rigid base removes any flex the frame is contributing. Flipping only works on genuinely two-sided mattresses, which most modern beds are not.
WHAT DOES NOT WORK. Plywood on top of the mattress under the sheet. Putting the mattress on the floor, which most warranties explicitly prohibit and which invites moisture problems underneath. Any topper laid over a mattress that has already collapsed — a topper follows the shape beneath it, so a soft topper on a sagging bed makes the hollow deeper, and even a firm topper only bridges a hollow up to roughly half an inch.
WHEN TO STOP FIXING AND START SHOPPING. If the mattress is still inside its sleep trial, return it and buy the right firmness — that is exactly what the trial exists for, and no warranty anywhere covers disliking the feel. If the trial has ended and the mattress is sound but wrong, a firm topper is the sensible spend. If the mattress is sound, wrong, and you are also fighting heat and edge collapse, you are patching several problems at once and replacement is usually better value.