Hi Allianna,
If you are looking for a new mattress then the first place to start is the mattress shopping tutorial here (it has a longer and a shorter version) which has all the basic information, steps, and guidelines you will need to make the best possible choices … and know how and why to avoid the worst ones.
Two of the more important links in the tutorial are post #2 here which has more about the different ways to buy a suitable mattress that is the best “match” for you in terms of PPP (Posture and alignment, Pressure relief, and Personal preferences) that can help you assess and minimize the risks involved in each of them and post #13 here which has more about the most important parts of the “value” of a mattress purchase which can help you make more meaningful comparisons between mattresses in terms of suitability, durability, and value.
There are some suggestions in post #4 here that may be helpful for a mattress that has developed soft spots or is sagging (or was too soft when you purchased it) and one of them is what you are doing which is turning the mattress over to the firmer side and adding a softer topper as a comfort layer but if you are sleeping on a firmer layer that was meant more for “support” than for “comfort” you may need a thicker topper than just 2" to isolate you more from the firmness of the support core.
Phoenix