To further shame's comment, a LOT of this is likely from the pure O, and "simply" anxiety. So many of the things you described are consistent with pure O and have been experienced by me an many others.
In particular your comments about loud thought chatter, intrusive "personalities," and even your fear of schizophrenia are all from the pure O.
I have intrusive thoughts, images, voices, etc go through my head often (thought form not auditory) and have finally realized after nearly 10 years of fear that I am not developing schizophrenia! And from the sounds of it NEITHER ARE YOU!! :)
Many people including us just have an overactive brain that does weird things which is definitely a pain! But educating yourself as you are doing now really takes the power away from the intrusive thoughts and makes them less fearful.
To recap, you are not going insane, you are not crazy, and you are not developing dual personalites!! Once you accept this fact then it will hopefully decrease some anxiety and help your situation overall.
On a different note, I too had issues with seeing therapists as they never seem to help come up with any sort of SOLUTIONS to problems. Their purpose seems mainly to be someone to vent to.
What I have found that helps most after literally 14 years of trying to help myself get better is the following:
1) Exercise (releases good chemicals AND boosts confidence) 2) Drastically reduce alcohol consumption. The alcohol anxiety lasts for many days or weeks after a binge. Quitting has really helped! 3)Take care of yourself!! Have a clean house, avoid negative people, and just generally love yourself and do positive things that will make YOU a better person overall. 4) Avoid getting lost in your thoughts. I don't mean avoid analyzing things, I just mean avoid going down the deep abyss of "why did I think that? Damn that was a scary thing to imagine! How crazy of a person am I? God no one else is like this, I am so fucked!! etc etc etc" 5) Maintain at least a small group of social contacts.
Lastly don't forget that there are SO MANY people out there just like us, and often doctors cannot find anything wrong (which should be considered a good thing!). You are not alone, but many of us eventually find peace, and from the sounds of it you are just dedicated enough to find your own sense of peace as well :)