What is the following in red saying?
If I choose to enter a room with a sense of joy and excitement, my positive mood will spread and will affect those present. And while I should certainly allow myself to experience painful emotions at times, in some situations it may be appropriate to "fake it till I make it" and imbue myself and others with positive energy.
Does it mean "pretend to be happy till I become really happy"?
Yes. This is not a new thing, but it's a newly fashionable expression. In The King and I there's the song
But Gertrude Lawrence, singing the year before I was born, would never have said 'Fake it till you make it'.Whenever I feel afraid
I strike a careless pose
And whistle a happy tune
So no-one ever knows