Fix it right now. Turn it in right now. Yes, that is harsh. I understand why plagiarism is treated severely, but that does seem excessive. Regardless of what she said, do it, and send a note explaining how you managed to overlook that citation. If you have anything else to mention in your own favor, such as that it was just one out of 100 citations and an oversight, say it (without whining). But take care how you express yourself, and do not accuse her of anything. Also document this correspondence. Keep everything, including the before and after versions of your paper. You will need them if you appeal this.