I'm really sorry you have to go through that. I'm lucky that my mom is generally supportive, although she still sometimes pulls the "why don't you comb it/do something with your hair" card. It's incredible that you haven't snapped at them yet, but the next time someone gets insistent tell them that it is YOUR body and YOUR hair, and they can butt out of your business. They might hate what they were born with, but that's on them. You think explaining that it was making your hair brittle and unhealthy would be enough, huh!
I agree with Lovemenappy about finding some kind of IRL support group or event where you can make connections.
Just keep at it, I bet when your hair is healthy and gorgeous these same haters are going to ask you about X Y and Z that you did to get it that way!