My standard piece of advice is to be clear about what you want in coming out to her. Just telling her you are trans may leave her in a mix of emotions (some good, some not very good at all). So you are trans - do you want to transition? How? When? To what benefit? What would you ask of her - to let you go on HRT, dress the way you want, cut your hair differently, see a counsellor, allow you to change your name... all of the above? Don't get into trying to justify yourself or explain why you are trans, tell her instead how it makes you feel when you aren't presenting as the gender you identify as - miserable, depressed, unfulfilled, or however.
Good luck! Hugs 🙂
