I’m not in your head to be able to comment with actual knowledge of your case, but a few things come to mind. 1) The mind has incredible capacity to do huge amounts of complex parallel processing with itself and the rest of the nervous system. You can think about several things at once, while doing other tasks, and with your subconscious doing yet other things, while processing vision, sound, music, and being aware of your surroundings, all at the same time. That involves probably millions or more pieces of information per second. Meanwhile, your mouth can say a few words per second, and processing that actually requires slowing down and keeping track of the words you are saying. 2) The difference in speed between the mind and the voice is huge, and one thing that humans don’t do all that well is short-term memory. So you may be able to do very complex thinking and then forget it before you can say it. 3) Converting thoughts into effective language is also a skill, which takes a lot of practice. 4) Sometimes ideas seem to make sense when your mind has a flash of inspiration (or especially, I find, while dreaming), but then when you carefully try to set them to words, you may discover they don’t actually make full sense under careful scrutiny (or you forgot or misremembered the brilliant parts by the time you got them into words). 5) Probably other things. :-) 6) Welcome to Fluther!