Even if your T had good intentions, she went about it in a completely inappropriate way. If I walked into a session and my T had a colleague there, unless I had given permission in advance, I would have left. Your feelings are very valid. I am shocked that a T would think it was okay to do this without consulting you first...kinda leaves me speechless

Writing a letter is an excellent idea...I would also consider going to T's supervisor (if they have one)

Good for you for not using SI to cope! You should be proud of yourself! That shows how strong you really are!