Suggest Remedies For Severe Hair Fall And Grey Hair

Hi sir/ma am I m 23, and I ve been having terrible hairfall from the past month, my hair is coloured but they have grown out on the roots. My issue is that I notice my hair has this white hair casts maybe at the root and It s like sometimes there are 2-3 hairs attached with that white module like thing. It s not the usual bulb, I can even attach pictures of it. I feel like itchy most of the time, and my hair falls like crazy everyday. I ve been to a derma and he prescribed me to mix n use sebonac and 8x shampoo every alternate day along with the multivitamins . But these shampoo tend to make it even worse. He told me its dandruff. But I m dicey. So I read about it alot, and found out it might be hair casts. And I don t know how to treat it. Please suggest me whom should I consult. I m so worried , might end up in depression . I m scared to use anything on my hair as of now.