I’m so glad you wrote. Yes! You absolutely should tell your therapist about your concerns. As you said, your therapist can only help you if you tell her your thoughts, feelings, and concerns. If you don’t share your worries with her, she may have to spend many sessions just getting enough information to assess you accurately when you could already be getting some good treatment.
If it’s difficult for you to share your concern, please just take your letter and this response to your next session. It will help both you and your therapist get down to what matters most.
I wish you well.