Hi shibamistress,
I would “echo” the previous comments … especially about memory foam.
Memory foam comes in hundreds of different formulations and in a wide range of densities some of which are “junk” and some of which are very high quality. You can see some of the things that make one type of memory foam different from the next (even in the same density/quality) in post #9 here and post #8 here.
As Novosbed mentioned, a topper is only suitable for a mattress that still has a firm even surface with no soft spots or dips in it that just needs some extra pressure relief because the comfort layers are too thin or too firm. They are not a good soution for “fixing” a mattress that is too soft or has developed soft spots or impressions because they will just follow the soft spots or impressions below them.
So if the issue with your mattress is that it is too firm and it has an even surface with no soft spots … then there are some topper guidelines in post #2 here and post #8 here that can help you make the most suitable choice based on your actual experience on your mattress.
Improving either primary support (the deeper layers of a mattress) or secondary support (the upper layers of a mattress) is much more difficult because it would normally involve removing or exchanging layers instead of adding to them and adding a firmer topper on top of a mattress that is too soft or has developed soft spots is not generally effective (see post #4 here and post #4 here)
Phoenix