Hello, I'd like to help you with your question.
Talking to yourself is something most people do. They do it in varying degrees but almost everyone does it. It is a way to sort thoughts and feelings. Talking out loud also lets you process issues differently which sometimes can help you solve problems. So it sounds like you have a very vivid imagination and that is probably all.
Since you have been seen by a therapist and/or psychiatrist, you should have been screened for all mental health disorders, Schizophrenia included. Your symptoms would have been apparent to any mental health professional evaluating you. And if you had Schizophrenia, it would have most likely shown up by now at your age. Typical on set for Schizophrenia is late teens, early 20's with some people developing it a little later in their 20's. Plus, the symptoms you describe are not even close to Schizophrenia. Add
to that your question, which is clearly stated and organized well, shows your thought pattern as clear. When someone has Schizophrenia, it often shows in how they write as well as how they voice thoughts.
You also mentioned that you are able to control when you imagine scenarios. If you had Schizophrenia or another serious mental health disorder, you would not be able to control when did
these things and how they worked in your imagination. People with Schizophrenia also have hallucinations and delusions, which are much different than imagination. They see and hear things that are not there and are convinced they are real. You know your imaginations are not real and that you are experiencing a vivid imagination. Someone with Schizophrenia does not have that kind of insight.
Over all, it sounds like you just have a more vivid imagination that most people. As long as you are able to see this for what it is and you can control when you do it, then you are just fine. If it does continue to bother you or you experience any anxiety
over it, consider trying therapy again, maybe with another therapist. It might help you to get a different perspective.
I hope this has helped you,