Hey there. Welcome to the boards. Happy to help you out with this. Let's see what we can do to help you feel less worried and make your exam tomorrow as positive an experience for you as possible. <3
Let's start by seeing if we can't make this all easier by not having your mother in the exam room with you. It would be really unusual for a doctor or other clinician to allow someone else in with you during a GYN exam unless you asked for that person to be there. But it does still sometimes happen.
Do you have the ability to call into the office and let the receptionist know that a) you're trans, b) you're not out to your mother, and c) you do not want your mother present in your exam?
Per the latter, do you feel able to just tell her you want that exam private, by any chance?
Never doubt that a small group of thoughtful, committed citizens can change the world. Indeed, it is the only thing that ever has. - Margaret Mead