I actually didn't used to have this "internal monologue". Until a couple years ago - when it just sorta showed up. And at some point I became consciously aware of it - and its ...power, I guess. Without the inner monologue it had been impossible for me to have "insincere" conversations. Using that word for the lack for a better one - I mean where I am talking to someone and consciously controlling what I say instead…
For me, I've always had it, and I can't control it either. however, it does go quiet in one of two circumstances, when I'm very, VERY relaxed (so, rarely) and when I'm deep in "the zone" of concentration, which is really nice.