I have a silly suggestion. Try sublingual B-12 with folic acid twice a day and a daily teaspoon of cod liver oil and tell me if you see an improvement.
I feel exactly the same way. My head feels empty most of the time. That is the best way to describe it. I do have thoughts and daydreams, but mostly they are the same inane thoughts and ideas over and over. It has come to the point where I don't talk to new people, as I have nothing to say. I struggle over words. I am marginally more confident of my writing ability, but even that is a struggle. I reread every sentence, editing as I write, because none of my words ever seem to go together. I read that is a form of depression. Sorry, I have no idea where I read that, so it may mean nothing. In my case, I wouldn't doubt that it was depression.
I wouldn't have believes such a condition existed, however I experienced such symptoms after being exposed to a very high intensity Tesla field of several million volts. For a week I didn't have a single thought pop into my head. Not a dream. I was very conscious of this. Nada. Then, normality returned after about three weeks.
The first thing I would do if I were you would be to have a consultation with a good psychotherapist. Your written statement here is full of rich and clear distinctions, and periodically you reverse the thought poverty, so it is very likely that what you have is psychological in orgin, and quite easily repaired.
For reasons that you can't explain, you have shut down and gone into a protective shell, and through the massive repression tha tis needed to keep that position, you have created the thought poverty. Psychotherapy is definetly your answer.