How can I make my hair grow longer and thicker?

I want my hair to grow longer and thicker. I used to have long beautiful curly hair but I straightened it everyday ever since I was in seventh grade and now im out of high school and im practically bald. well not really bald but my hair is extremely thin ! and its shorts and it doesn't grow and its not even curly anymore;/ ! my mom says it doesn't grow because im half black. which I don't think its true because it was really long before. please help?