Hi dondro,
A topper can be a good idea if a mattress is too firm and still has an even surface with no dips or soft spots and only needs some extra thickness or softness on top. If your mattress is too thick and soft in the top layers or there are soft spots or dips in the surface then a topper is not a good idea and can even make the problem worse because it will just follow the dips or soft spots in the mattress and with even more soft materials on top could allow your heavier pelvis to sink in even further than it already is which can put your spine out of alignment and lead to back issues.
It sounds to me like your mattress is at or past the end of its useful life (many pillowtops that generally use lower quality materials in the pillowtop don’t last nearly as long as yours has) although there are some suggestions in post #4 here which may provide some temporary or partial help for a sagging mattress or one that has developed soft spots and lost its ability to support and keep your spine in good alignment. Post #4 here and post #2 here may also be worth reading about why adding a topper won’t generally “fix” a mattress where the foam has already softened and is breaking down.
If you do decide to go in the direction of a new mattress then post #1 here is the best place to start.
Phoenix