I have really, really, thin 3b hair that doesn't grow past just above my boobs, what can I do?

since I was little I had thin hair, but never this thin, it breaks too often and I have never been taught on how to care of it. My hair type is a really soft and thin 3b (I've been told) and from the age of 15, I started to dye it and I straightened it almost everyday (now 21). But now I want my hair to thicken up and grow, but I don't know how and what I need for my type of hair.I haven't dyed or straightened my hair since I was 18, so that's not the problem.My sister's hair is a 4b and the routine and hair care she does, doesn't work for me since we have different hair types. Please advice me on what to do and how to regrow and really thicken my hair. Thank you so much in advance

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