This really isn't a forum to complain about your helper on.
She is an employee and a member of your household. You are an adult. If it isn't working out you know what you need to do. Your complaints are so vague that it makes it difficult for anyone to give you any helpful feedback.
However, if you really don't know how to communicate with your employee, here are a few suggestions:
(1) Put your expectations for her job down, in writing. Be as detailed as you think you need to be. Have her read it and speak with her about it, when you are calm, and ask if she has any questions.
(2) Regarding whatever it is that you think she is doing incorrectly, first, make sure she understands how to do it right - show her, ask if she has questions, gently correct her the first few times.
(3) If she makes a mistake, again calmly, ask her why she has made whatever mistake (repeatedly), after you have explained to her how you want things done. Listen to her answer. She may genuinely not understand what you want from her, or there may be some other reason - is she overworked, is she getting enough sleep, etc.