I guess there is a bit of a vicious circle going on here. The more your parents shout at you, the more "mistakes" you make and the more "mistakes" you make the more your parents shout. The more you're shouted at, the more stupid you feel, but you are NOT stupid, you are just reacting to a stressful situation. If you can figure out a way of breaking the cycle then perhaps things will improve, being yelled at and verbally abused isn't healthy for you or your parents. That's where a counsellor might be able to help you.

There is no such thing as a normal teenage girl, and I'm sure you're fine just the way you are, that's something for your parents to work on and not for you to change.

I think that the problems you're having need to be dealt with as family, but if you can't see that happening perhaps you just need to spend as much time with your friends as you can, putting a little distance between you and your family might just relieve the tension a bit. Do you get on OK with your brother, can he support you in all of this?