Multivitamins are basically guesses of what a typical person might need. They are not at all customized to your biochemistry.
Research has found that patients with ME/CFS have imbalances in biochemistry and in their microbiomes (which may affect nutrient processing and absorption).
If you are depleted in one or more nutrients, you may have biochemical pathways that are lacking what they need to run properly. This may cause a backup of high levels of other nutrients as they are waiting for what you're missing in order to be used. And some nutrients, like B6, are used in multiple pathways, so being short of it can cause multiple problems.
And, suddenly adding in nutrients can contribute to an overload of the things that are backed up, or you suddenly take the bottleneck out of a stuck pathway, and then move along to the next bbottleneck where you're missing something else.
Most, if not all, people are toxic to some degree. If you've been short of nutrients used in detoxification pathways (like B vitamins, and certain minerals and amino acids), adding nutrients can begin to mobilize toxins (arsenic, cadmium, lead, mercury, organophosphates, etc.) which can produce unpleasant symptoms as well. (I once started a very good supplement, under a doctors supervision, only to have arsenic be mobilized and measurable at a level the CDC considers acute toxicity....)
The best way to sort out what you need is to have a comprehensive nutrient test, like a Genova Diagnostics NutrEval run. You can find it at
www.gdx.net, as well as finding a doctor who can run it.
Then, you can develop a nutrition plan that's customized to your needs.
Other tests that might help you figure out what's going on might be a heavy metals test which shows if you're toxic and with what, or a DNA stool test, which tells what bacteria are in your microbiome, which is useful as certain bacteria convert certain nutrients from what you eat into forms your body can use, and without them, you may not be able to use the nutrition you eat.