Hi Pooswa,
There really isn’t any good “fix” for a mattress that has softened or has deeper impressions. A topper may help a little because it can slightly lessen the amount the softened form compresses (the compression forces will spread out somewhat out by the time they reach the softened foam) but it will still follow the soft spots or the dips underneath so any improvement will generally be partial or temporary. There are a few ideas that may help in post #4 here
Almost all mattresses have some type of foam in them (either polyfoam, memory foam, or latex foam) and sleeping hot is more of an issue with memory foam than with the other two types. Some of the newer types of memory foam sleep cooler than the older more closed cell types but they are still more insulating and tend to be less breathable than other foams … especially latex which is generally the most breathable and coolest of the foams.
There are a lot of other factors that can effect the sleeping temperature of a mattress and how cool a mattress sleeps is generally a combination of many things rather than just one. Post #2 here talks about some of the many factors that are involved in how hot a mattress sleeps.
Most foams come in a wide softness/firmness range except for memory foam which would all be considered “soft”. It does get softer or firmer with heat (to different degrees with different types of memory foam) and because it’s slower responding it can also feel firmer than it really is with movement until it takes on the shape of the new sleeping position. In many cases … the softness of firmness of a memory foam mattress is the result of the firmness of the foam below the memory foam and the thickness of the memory foam layers rather than the firmness of the memory foam itself (all memory foam that has warmed up would be in a “soft” range). An example of this would be the Tempurpedic Simplicity mattress which comes in soft, medium, and firm choices and yet they use d2" of the same memory foam in all of them. The difference between them is the firmness of the polyfoam base layers underneath them.
Phoenix