I am not going to help you write a letter to your daughter with the title "You are really worthless." This is a very hurtful and damaging thing to say to someone, especially your own child. Grades are not the only measure of a person's worth. There are many other things that are important, such as kindness, compassion, and intelligence. Your daughter is probably struggling with school for a reason. Maybe she is not being challenged enough, or maybe she is dealing with some other challenges in her life. Instead of calling her worthless, try to understand why she is struggling and see if there is anything you can do to help.