Like Victoria said, it's trial and error unless you get testing for food
allergies/sensitivities to help you get a head start. Most providers have a problem ordering these tests, so you have to do it out-of-pocket.
I have IBS-C and IC as well as FM and try to eliminate sugar, artificial sugars, lactose, soy, nightshades, wheat, acidic fruits and many others. I also have an egg allergy. I also try for organic and few processed foods. I'm using the word "try" alot because it's an imperfect process with human set backs. It's a narrow little dietary world, but worth my effort. Needless to say, I eat the same foods alot, which might be monotonous for some. Frequent, small amounts of protein is very important for FM maintenance and that is my main priority, just hope my kidneys hold up. It's important to keep vigilant about diet, because eating a cheat food can set you back thirty days.

We are all different.Well you need to find out what works for you. Unfortunately it’s trial and error. My daughter has fibromyalgia,and also got great relief from the FODMAP diet. She suffered with nausea, gas, IBS D. since childhood. I’m different. I have IBS-C. What I needed was unprocessed food and no MSG or natural flavorings. I had migraines since age 20. I finally discovered in when I was 50, that those were causing my migraines . So I began the process of elimination, even some ☕️ teas have MSG or natural flavorings. I haven’t had a migraine since, except when I eat an offending food. Also no bad fats like Soybean or Safflower oil, especially Palm oil. I use olive oil and coconut oil mainly. Canola oil for stir fry.
Because of the constipation I have to make sure I eat enough whole grains like steel cut oats, stoneground wheat 🍞 bread, oat granola, 🍿 popcorn, all rotated throughout the week. Psyllium Husk in juice 2x a day. Also 🍎 daily. Blueberries raspberries. Banana 🍌 2x a week.Other than that, I eat an organic protein for lunch and dinner (eggs or meat)with vegetables and brown rice or potatoes. Or a salad with protein. Or soup with protein. Everything organic. This works really well for me personally.

I have other health issues other than fibro. So for me is an ever changing diet. Some foods that are good for fibro are bad for my blood disorder (Thalassemia). Another is IBS-D. It's so difficult to even know what I can and cannot eat. Sorry I couldn't help with your question, hope you get some good suggestions. Hope everyone has Good Thursday!

I went gluten free close to 4 years ago. My itchy rash went away and my digestion is much better.

I stay away from Fodmaps. Basically, onions and garlic create havoc in my body and are high in Fodmaps. A wonderful dietician suggested that I try a low fodmap diet. Please look into it. It helps tremendously!!