Writing the email was cathartic, but deleting the email instead of sending it shows maturity and good judgement. The email sounds immature, and I am certain that is not your intent. You are working full time subbing - how much of an impact do you really think this person is having on your success? Be the bigger person, or the adult, and act accordingly. The email could make you seem weak and petty, and I am sure that is not your intent. Write as many emails or letters as necessary for you to feel vindicated - never send a single one of them. You will be grateful in the end. Best of luck.