You could simply say, I'm not sure this is the right fit for me. You don't even need to go into details. If you feel it would be awkward to say in person, you could simply email it to her, leave a voicemail, text, however you're able to communicate with her. If you don't want to be without a T until you find another one, maybe start the search now, then you could keep seeing her until then. But I think you said you were considering someone in her office, right? If so, then I'd go ahead and say something to her, that the fit doesn't feel right and you'd like to try someone else.