I have a great relationship with my mother now but she is my BPD kryptonite. She stresses me out more than anything else in the world and fundamentally doesn't understand the concept of boundaries. I still make sure there is zanax in my purse before we go out to dinner.

If you are under eighteen, in the US your therapist might have to report the abuse legally. That said, I used to volunteer as an advocate for victims of assault and in the real world therapists and advocates know full well that if a victim doesn't want to participate in a complaint it is: 1) pretty much an impossible uphill battle and: 2) can really get in the way of actually helping.

You should be able to talk about what happened to you. If you are nervous ask the therapist hypothetically "IF a patient my age told you about something LIKE____ that happened when they were ___ years old, would you report it against their wishes?"

That way the therapist can give you an idea of what rules they have to obey but doesn't have really have enough information to go against what you want.