I learned the repetition and recombining of speech sounds before I learned what words were for. The result us that speech developed for me not as "I will say what I am thinking" but "I will say something that at least sounds plausible in reaction to whatever was said, and/or I will get everything I am going to say by mentally cutting and pasting from the words of others whether spoken or written." The result is that by the time I knew what some words meant and what wordsw ere for, my brain had already created a thing I called the "Translator" which came up with words independently of my thoughts. So when I tried to say what I meant, I had to wrest control away from the Translator. Which almost never worked even when I could speak.
Writing started out similar to speech but for some reason I had more control over it than the Translator did. So that combined with a loss of the ability to speak most of the time meant that I now use writing for everything. But every now and then, especially when stressed or tired, the Translator still slips some words in... especially since I rely on something very like the Translator to supply me with words in the first place.
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"In my world it's a place of patterns and feel. In my world it's a haven for what is real. It's my world, nobody can steal it, but people like me, we live in the shadows." -Donna Williams