If you are a minor, probably not a whole lot is confidential by law. I think you do have to discuss this issue with your T though or there's no point in spending the money on this T, if you feel you cannot trust her to tell her what you would like to. I would have issues with my T not telling me that she is telling my Mom. That doesn't seem completely open and honest to me and I would be tempted to respond the way you are, to then not tell her anything. That's not a wrong response on your part, I think it is a mistake on your T's part.

I'd put down a boundary and let your T know if she wanted to tell your Mom something she would have to tell you she was doing so; if you wanted your Mom to know some of this stuff, you would just tell your Mom and keep the money! Tell your T you won't be able to keep seeing her if she's trying to play both sides of the fence, that's disrespectful to you and not something you want or need.