Do you want the T to know what is in the letter? Bring it with you and read it to her. She does not know what you are going to do; that is up to you! She only knows what she prefers and she gets a vote as it is between you and her. Demanding another person do things in your way is an attempt at boundary crossing. It is not wrong or bad, but the other person gets to say, "no thank you, I prefer this way" if it concerns them. If someone gives you a gift, you can throw it on the floor and stomp on it or take it away and throw it in the trash privately or whatever you want. T has temporarily returned your gift to you and asked it be given to her in an appropriate therapy manner. She is trying to help you by teaching you how to give and receive, how to successfully interact with others.

She did not have to send the letter back to you, she could have just given it back and asked you to read it during the next session, where you would be on the spot or could have just "told" you she did not read it and would not and left it at that (or sent it back without a note!). The note was helpful, so you know she does want to hear what you have to say but in the appropriate fashion for therapy.

Therapy happens during sessions, when you are face-to-face with your T and you respond to her and she to you; it cannot happen here, it cannot happen in a letter, therapy can only happen in therapy, when all of you is engaged with your t.