First, I send you my love and caring, which you need because you're in such a difficult situation. I have to say that reading your letter threw me back to my childhood. I have been hyper-active all my life and was recently diagnosed with having ADHD. When I was in elementary school, some teachers enjoyed having me in their classes and some couldn't handle my energy. I'm very creative--an author, artistic, etc., but have difficult times sticking to tasks--For example, I should be working on an important project instead of writing this letter to stick up for your little girl when you're so angry at her.
My mother came from a large family where people weren't allowed to be different or creative because they needed to work. My mother was not allowed to use her artistic talents and resented my interest in everything and my sensitivity to life, which she tried to squash. She had very little patience and would get frustrated and angry. She talked about me to other people right in front of me---like I couldn't hear her or like she wanted me to hear her so I'd be embarrassed and shamed. I wonder whether your daughter is feeling your frustration and anger and is feeling frustrated and angry back--that's the reason she's hitting you.
Anyway, you've received some great replies from the parents' point of view, I thought I'd fill you in on the child's point of view. Remember that you're dealing with a thinking, feeling person. I think the only way to make this problem better is to get some help yourself so that you can be more understanding and patient.
I can't imagine why you want to have another child when your daughter needs your love, kindness, and understanding right now. How could you handle another child without leaving your daughter to drift off into nowhere? This makes me feel like you've given up on this little baby, and you want to start over with another one. Be a good parent to this child, and then have another. You'll be rewarded with a wonderful daughter who loves you instead of a wild girl who may get into trouble. I was too afraid to get in trouble, but things are different today. Please help yourself and your daughter. I'll be praying for you.