Something about your idol’s change in behavior and personality really, really confused and distressed you when you were young. By imagining that you are her and talking to her friends, you are deeply involved in trying to solve that puzzle. You have quite literally tried to “walk in her shoes”. Unfortunately, what started out as an attempt to understand has become a preoccupation.
You’re correct. This isn’t helpful to your growth and development as an individual. Even if you somehow did come to understand your celebrity idol’s issues, doing so isn’t going to help you be all you can be.
If you could stop the preoccupation, you would have done so by now. I strongly suggest that you make an appointment with a mental health counselor for some therapy. A counselor can help you reclaim you “self” and will help you learn from the experience. Take your letter and this response with you to the first therapy appointment to help the two of you get started.
I wish you well.