Wow. This is so much my experience too. I often build my T up in my mind out of session. I imagine she is the perfect mommy I always dreamed I would have.

And then, when I see her in person, I see that she is a good, kind person--but completely human and imperfect. When I'm sitting with her, I see that she is not the perfect mommy after all.

I think it becomes dangerous only if I choose to live in the fantasy and not accept the reality. But so far that hasn't happened. Usually, I'm able to take from her some of the good things I fantasize about--her soothing voice, a warm hug, the way I feel safe in her presence--and I cherish that as being reality. I think that is real.