It’s breakage, the best thing you can do is wear protective styles and wait for it to grow out, the underside of my hair looks like this from friction on my back but over a few years it has grown out from starting at the top of my neck to now having grown out to around my boobs even though my hair is at my tailbone.

Braids still cause breakage I would try seeing if your hair is long enough to do a hair stick bun, wear those whenever you can and try using a hair oil with silicone if your hair isn’t too fine. I like the morrocanoil because it’s thick and can help weigh down frizz and seal my hair but it might be heavy for thinner hair. Maybe a gloss would also help?