We never really know anybody. Most of the time we accept this easily and naturally and it doesn't cause much in the way of issues. The fact that therapy makes a client 'feel' known, and makes them 'feel' close to the therapist is one of its many manipulations and lies.

I suspect the reason why this feels so powerful is because it reminds you that you don't really know this person at all, and no matter what happens in your sessions, or how much you tell her, you never really will.

And that includes what she really thinks about you when she's not being paid to put on a facade.