Because something isn’t usual or isn’t usually known doesn’t mean that it is a symptom of a mental illness. There may be nothing “wrong” with you except that you have a talent or a way to solve problems that isn’t well publicized.
Many people talk themselves through a task or a problem. Sometimes it is audible to others. Often, they manage to keep it to themselves. Many people create scenarios and stories as a way to think about people’s interactions with each other or to problem-solve. Sometime people who practice dialogue this way become novelists or screen writers or play writers. Others simply use their well practiced skill to entertain their children and friends.Your letter suggests to me that you are an especially creative person who, at a young age, happened on a way to entertain and help herself.
I am concerned that you are causing yourself a great deal of distress by looking for a diagnosis. I’m not as concerned as you are. You know the difference between what’s real and what isn’t. You aren’t hearing consistent voices in your head. You report that you are not having hallucinations or delusions. From what you said, your story telling isn’t hurting you or anyone else.
I can’t, of course, discount a mental health issue on the basis of just a letter. It’s always wise to get concerns checked out — if only for peace of mind. If this continues to be a concern, do consider an evaluation by a mental health professional to settle the question once and for all.
I wish you well.