Thin roots and hair isn't beautifully thick...what to do?

Is there any oil that can fix my roots and hair? my hair is unhealthy, you can tell by looking at it. I do a bun and the area where i tie it, near the crown of my head, i feel due to tying the bun there the hair is EXTREMEMLY thin from there now! what's wrong with my hair? I think i am anemic, does that have anything to do with it? my scalp has extrememly thin hair as well...and as for my hair that falls back, it is okay but thin compared to what i used to have...i know straightening and moussing hair is damaging but if i dont do it anymore (for almost half a year)...why is my hair still thinning, especially from the crown of my head now?