Fibromyalgia is not accepted as a real thing by around 50% of medical professionals so you have that barrier to surmount in the first place. This is because whilst some medical professional insist it is a "thing" of itself, most believe that it's a collection of symptoms given a label when differential diagnosis cannot come up with another explanation.
Starting from the assumption it's definitely a thing means you risk being viewed as lacking credibility to begin with.
However, the bigger issue with fibro claims is that people tend to err on the side of saying how difficult it is to explain instead of being very specific about which points they should get and why. Focus on regs 4 and 7 and not how hard it is to explain. That goes nowhere fast. All conditions are unique and our experience of them is also unique to us. All conditions are variable and again that's not unique to fibro. None of these things should make it hard to identify which points and why.
These regs are about the fact that needing help more than 50% of the time means you need help, and, that things you cannot do reliably, repeatedly, safely or in a reasonable time... can be recorded as things you cannot do.