I have never been a fan of that saying. It reeks of dishonesty or of being disingenuous; not traits that I wish to embody. Yet it frequently is deployed when discussing how to manage stage fright.
Okay, what about:
Mood follows action
That, I would submit, is what 'Fake it 'til you make it' really means, and it isn't dishonest at all. This is especially true when we couple our actions with our values. All of these elements together, bring us closer to our goals, in spite of our performance anxiety.
Let's say that I wish to express great drama at a certain point in a song. This is my stated goal.
I ask myself, what would someone who was confident in their performing capabilities do at that moment to convey the sentiment of 'drama'? They might extend their right (or left) arm, in a sweeping upward fashion curling their hand and holding it upwards towards the heavens.
If that action is in alignment with my values, then I can execute that specific motion at that moment in the song, and this next part is key, whether I am feeling confident or not. So long as the action aligns with my values as a performer, I can execute it.
In this way, we bridge a gap in our performance anxiety. We're taking an action that we identify as being one that a confident performer does and executing it ourselves at the chosen moment in the song. It is likely that this will feel funny, or disingenuous initially. With time, and practice, however, these externally motivated and chosen moments can become internally motivated and more spontaneous. It just requires us to bridge the gap between ourselves as performers with performance anxiety, and ourselves as performers who are managing our performance anxiety.