Girl, my mother said the same thing to me...."I don't know why you think you can grow long hair! You ain't never had long hair! I ain't never had long hair! So, what makes you think you can grow it long now?!"
I was like "Maybe I never had long hair because no one was taking care of it for it to grow? I know it grows, or else I wouldn't get a relaxer every 6-8 weeks. I'm just trying to hold on to what's already growing" (I was another one relaxed at an early age...6, I think)
Anyway, about a year later, she's all up in my face wanting to know what I was doing to get my hair so long
"Oh? What? This stuff that you said wouldn't grow?" She's gotten a lot better now, btw. She's even started stretchign her own relaxer and asking me about products.
My mother isn't controlling, though, she's just a nut!
Grow your hair and go on about your business. She'll be alright. Trust me.