I like your description of how your body language changed. As to whether it's dangerous, I'd say the question is a little broad.

Let's take the earlier example from The Game. I would argue that the false belief version you present (most attractive, etc.) can be a useful counter to Bruce-programming (self-defeating behavior), but that it is not necessary or even optimal to have a false belief of one's status (except perhaps as a training stage) to exhibit attractive body language. But maybe that's beside the point, because I would not be surprised if some aspects of our lives that are outside our direct conscious control (like our overall body language) could be optimized in some situations by having, or as you put it, rehearsing false beliefs. But I doubt those situations require more acceptance of the false beliefs than acting does. So it wouldn't be a "tiny awareness" recognizing that it's a lie. I'm not sure how to characterize it--a different self, a sort of meta-lie, but that kind of "mind hacking" doesn't seem like a path to the dark side inherently. In rehearing a false belief for acting, one is basically required to recognize it as a false belief, but one could imagine someone stumbling upon a false belief (say, that they're the most attractive person on earth) unconsciously (i.e. not by reading the Game or being fully self-aware that they're adopting such a belief, esp. say in the teen years) and then they get positive feedback socially and then when called on the false belief later, have trouble stepping back and being objective. But again, that's a different situation than rehearsing a false belief in a self-aware way. This doesn't rule out such rehearsing, as in acting, could have negative effects. Daniel Day-Lewis is famous for immersing himself in his characters--such methods should show negative effects if there were some. Do actors who use such methods show long term tendencies toward irrationality or some such that actors who don't do such immersion don't? I doubt it but don't have the data to say one way or another.