Yes, same as lonesome, I've totally done that thinking of my T! ( and various other people...my old CBT T I could have written a book!). It's okay. You're not hurting anyone in real life, I think it's a safe way to work through these feelings. If you haven't actually hurt her, or anything that belongs to her etc...it's all okay.
I understand your struggle with this, but please don't think of yourself as a bad person. There's a thread on here, 'fantasy session with T', have a read and see how usual it is to feel this way.