Hey there, is your therapist a speech-language pathologist (if in the states)? As an SLP that stutters myself, that diagnosis of cluttering turned stuttering is pretty strange. I would want to really make sure I was doing a thorough differential diagnosis, because why they can co-occur, it isn't very common, and they are different fluency disorders with different therapeutic approaches. Cluttering doesn't just turn into stuttering. Practicing saying words in the mirror is also a pretty outdated approach that isn't very effective when your goal is to improve ease of communication. Any chance you can find someone who specializes in fluency? You may need to switch therapists.