Hi paisley,
Actually I don’t think this was nearly as big a mistake as buying a mattress that was too soft would have been. It’s relatively simple and inexpensive to add to the comfort and pressure relief of a mattress that is too firm in the comfort layers by adding a topper but it’s really not possible to effectively “fix” a mattress that is or has become too soft because you would have to cut it open and remove the layers that are either too soft or have softened and degraded prematurely because a topper will just follow the soft spots on a mattress.
If your mattress still has a firm and even surface and hasn’t developed any soft spots and you are looking to add more pressure relief to the mattress, then it would make a good base for a topper. This also has the advantage of adding to the durability of the mattress comfort layers below it (which are the weak link of most mattresses) and it’s much easier and less costly to replace a topper than it is a whole mattress when the top layer (the topper) wears out sooner than the other layers in your mattress.
The choice of the best topper would depend on what was already in your mattress comfort layers (all mattresses will already have some softer comfort layers), your body type (lighter people need softer and possibly thinner toppers) and sleeping style (side sleeping needs thicker comfort layers than back or stomach sleeping), and which materials you were familiar with and preferred (polyfoam, latex, memory foam, wool are a few examples) but the topper guidelines in post #2 here and some local testing on different materials so you know which you prefer along with conversations with manufacturers like Brooklyn Bedding will all help you decide which is the best choice for you.
Again … because you are lighter than average … softer toppers will generally be more effective for you than someone that was heavier where a firmer topper would feel softer because they will compress it more with their higher weight.
Another advantage of buying a topper from a source like Brookyn Bedding or other manufacturers that sell component beds where you choose your own layers is that if down the road you choose to buy a mattress that includes a layer that is the same thickness and softness as your topper … many of them will let you use the topper instead of one of the layers of an equivalent thickness which will reduce the cost of the mattress.
Hope this helps.
Phoenix