Is it relatively easy for a girl to grow her hair back after shaving her head?

yes! Actually, your hair will grow a lot better after you cut it. When you do cut it you can start to solve some of the problems you have now so that your hair stays healthy. Start to use natural products. Stay away from grease because thats a common misconception with black people. Use natural oils to oil your hair like olive oil or coconut oil. Also, start co-washing your hair. (thats washing your hair regularly with conditioner; black people can wash their hair as much as they want as long as it's not with shampoo.) Only shampoo your hair once a week. Do this, and I promise your hair will grow!