So the idea behind "fake it until you make it" is basically even if you don't feel positive, change your thinking and it changes your life. To that, I would agree. I would like to change the tone of the sentiment slightly, however.
Neuroscience teaches that instead of "faking it" we are actively constructing the reality we choose. Our attitude can be changed simply by thinking about it . Our brains believe what we tell it . Simply by thinking a thought we create change in our bodies. Our neurons fire and create connections. Repeat the same thought often enough and our magnificent brains figure that thought is important and dedicates more resources- more neurons- to that thought process. Over time our bodies create neuro-pathways and nerve clusters just to help us process our common thoughts more efficiently. We reprogram our nervous system just by how we habitually think.
When we think positively even when outside circumstances may lead us in a different direction we are actually changing our bodies to make what we think real! In effect, simply by changing the way we think we change the reality we experience.
So instead of "faking it" we are actively constructing the reality we choose. Sometimes it takes a bit of time before the changes we make internally are reflected in our daily lives but we are not "faking" our positivity. We are training ourselves for a healthier, brighter future. We are actively choosing our path.