Hi MrMattress.
Welcome to our Mattress Forum. 
Sorry to hear of the premature breakdown in the foam of your mattress. It is most likely due to a low quality memory foam in the comfort layer. Our recommendation with regards to memory foam for someone of a normal BMI is 4 lbs per cubic foot. You can read our Durability Guidelines here.
It’s generally much more difficult to fix a mattress that was too soft initially or that has developed soft spots or is sagging because the best solution would normally be to open up the mattress and remove the layers or components that are too soft or have developed soft spots or are sagging and replace them with new layers. A firmer topper will generally just “follow” any sagging or soft spots underneath it and at best it may be a partial or temporary solution and at worst it can make any issues that you are having with your mattress worse. For example if you add a firmer topper you could end up with a sleeping system where the top layer (the topper) was too firm for you to provide good pressure relief but the layers underneath your topper could either be too soft or sagging too much to provide suitable support to keep your spine in good alignment.
There are some suggestions in post #4 here that may be helpful for a mattress that is too soft or is sagging at least on a partial or temporary basis but I would keep your expectations low because the odds of success wouldn’t be very high … particularly over the longer term.
I see you have posed questions in another post which I will address in that thread.
NikkiTMU