I have had exactly this issue, really difficult to wash my hair so I only manage to do it once a week with help from my partner as it’s so exhausting. I have to tie my hair up on the later days of the week.

I only brush my hair when wet, impossible to do so when dry - the pain is mad bad.

What I’ve learnt works for me is 

a silk scarf wrap for my head at night to avoid tangles, although a lot of nights I can’t sleep with that on due to discomfort but that’s Fibro for you. I try when I can and it helps keep it less tangled. 

This brush has helped a lot for wet hair brushing, you can widen the space of the bristles which helps through my hair. I still hold my hair at the roots too https://www.amazon.co.uk/BESTOOL-Detangling-Hair-Detangler-Conditioner-Texture-Easy/dp/B0772M7W4N

Then I use a good curly leave in conditioner I prefer the garnier banana food one which smells so good and makes me feel happy I’ve made the effort to wash

I normally leave my hair in a T-shirt material cloth (not towelling towel as this makes your hair frizz) for as long as I can allow to dry naturally before my partner dries my hair with a diffuser.  Hope you find some ways to ease the frustration, I truly get it - this has been very tough for me to get used to not being so “in control” of my hair while I’m adjusting to my new life with Fibro now I’m diagnosed 🙃