The most important thing you shared in your letter is that you aren’t telling your therapist about your thoughts. Your therapist knows you. I don’t. Your therapist can help you get to the bottom of these thoughts. I really can’t. All I can do is make a few guesses.
For that reason, I’m going to focus on why, maybe, you aren’t using your therapy well. I wonder if you are avoiding life (and maybe handling anxiety) by thinking about things that are forbidden. Think about it like this: You have two books to read: One is about the exciting life of a murderer on the lam who is using drugs and hiding out in a psych ward. The other book is about the ordinary life of a teen-aged girl who is going about her business doing well in school and enjoying a social life with friends and getting excitement by going on roller coasters. Hmmm. Which book are you more likely to want to read?
The problem with those scenarios is that an “ordinary” life of a teen is seldom that ordinary. Learning how to get along with others, how to manage the demands of school, and how to find exciting things to do that aren’t anti-social is challenging stuff. It’s easier to think about secret forbidden thoughts than to actually do the “work” of being a teen. That “work” is figuring out who you are and how to get along in the real world.
I suggest you take your letter and this response to your next appointment with your therapist. It would probably start one of the most important sessions you will have.
I wish you well