You didn't do anything wrong. Your therapist wanted to hear what you had to say in your own voice, with your own emotions there.

You read it out loud and I think you were very brave! That was hard, as you say. Doing hard things builds self-esteeem. Be proud that you summoned up the courage to read your letter in session!

Even the BS'ing about the weather afterwards speaks. It tells of your discomfort with continuing talking about the things in your letter. I think your T was giving you some space and time and some distraction after the stress of reading. Tell him next time how you felt about the part of the session after you read the letter.

There is no grading or judgement involved here. Your experience was your experience.

I think you did perfectly fine!