Nathan, I have that too!
I often feel very foolish when I am in a conversation and can’t recall the key word for what I want to say. Somehow it happens especially when the subject is something I am passionate about or have read a lot about. I recently thought it might have to do with the fact that I read a lot but don’t really talk a lot. So the word is in a passive memory, not in an active memory. Again, there are specific words to refer to those memories that I can’t recall at the moment! 
I used to limit myself in social outings as well, out of fear of shutting down.
What does help is my (small) doses of methylfenidate, because now at least I don’t lose the rythm of the conversation, even if I can’t find the right words.
I am considering doing the NaNoWriMo in november, just to get over my fear of writing. A month of just busting out 5000 words a day, no editing, no reading back to see if it makes sense, just throwing it all out. Who knows, maybe after that the active recall memory is a bit more operational…
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