Hi dbunny1,
Hopefully you’ve read the tutorial post which has links to the most important information and posts you will need so you can keep your reading to the parts that are most important.
The sagging you are talking about usually comes from using low quality/density materials in the comfort layers of the mattress. This is why it’s so important to make sure you know the quality of the materials in any mattress you buy and only deal with manufacturers or retailers that will tell you. The type of material or the style of mattress you prefer is a preference but all materials have lower and higher quality versions so it’s important to make sure that your mattress doesn’t have any obvious weak links.
It’s not so much the pillowtop but the quality/density of the materials inside it that can be an issue.
With fibromyalgia your mattress choice can have a big effect on how you feel and it’s especially important to make sure you do some good testing to make sure that your mattress is a good match for you in terms of PPP. A forum search on fibromyalgia (you can just click this) will also bring up some feedback from other forum members with the same challenges that may also be helpful.
Phoenix