On the one hand, making yourself smile, laugh, and stand in confident posture has been shown to make you feel and act happier and more confident. Action breeds action, practice makes perfect, and all that.
On the other hand, acting in a way contrary to your true feelings is draining and doesn’t feel very authentic. Besides which, most people can spot a phony smile.
I think the best approach is to start on the inside, understand yourself, so your actions can be aligned with who you are in a positive way, even if the actions feel uncomfortable at first.
For instance, any introvert in the Western extrovert world can attest to the pressure to act outgoing, speak up, and spend a lot of time with other people. There are situations where I force myself to act this way to network and represent my employer. Though I have learned most situations don’t require me to act like a different person, I’ve had to find a way to behave appropriately when I don’t feel like it.
What I do is think about the person I want to be and why. I want to be friendly and considerate of others because those are characters I value highly. This way, I’m pushing myself out of my comfort zone toward a better skilled, more successful version of myself – not a desperate version of someone else.