You briefly mentioned non-verbal autism, and you got really close for what it seems to be like for my son. His vocabulary is HUGE, but his ability to express himself by stringing those words together is limited to a few small 3-4 word phrases.
When he is forcing to get words out, I can tell he knows what he wants to say, but gets frustrated that he can't get it out of his mouth. When you said they must feel like "Why can't I just get it out! Come on!" (I'm paraphrasing, I think.) you're spot on, in my personal experience.
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