[imaginarybird]

I don't know what type of doctor you'll be seeing (pediatrician vs. general practice etc...) but in any case it has always been my experience that they have a bit of time where they won't have your parents in the room so you'll feel more comfortable being honest with them. Given that this will be a new doctor and I would assume you'll have to do a lot of background information and history, this will probably be before they even get to much of a physical exam. If you're really worried about your mom finding out, you might use that time to just tell the doctor "look, I did this, but I don't want them to know". As long as it is no longer a danger to you, the doctor will not be able to say anything, and will respect your wishes about not letting her see.

I hope it works out for you.