Your letter shows that you really are an intelligent and thoughtful young woman. Please relax. It is not unusual for people to make a running verbal commentary about their life. Everyone, in fact, does this silently all the time. Often, it is a way to solve problems or to get organized or to track what is happening. Often talking out loud helps us slow down and think something through more carefully. Most people eventually internalize the process or confine talking out loud to when they are alone — as you do. It is only a mental health problem if it is a product of hallucinations — which you say you don’t have.
However, if it has become a habit that you are having trouble breaking, you may want to see a counselor to learn some new ways to monitor yourself and to confine your tendency to speak aloud to when it is socially appropriate. Counseling may also help you address why you are so lonely and shy. Be assured that a counselor will be sympathetic and won’t judge you. Take your letter and this response with you to the first session as a way to get started. You deserve to be more comfortable with yourself. And you deserve to be less lonesome.
I wish you well.