In life people have a tendency to in-part (and some people largely) define themselves by what befalls them in life rather than what they themselves do. They develop a victim complex where they become their plight rather than their choices. Life is horrible and full of suffering. And there is oppression and exploitation of various dimensions of our world. But people can at-least potentially fight back if given the right materials - the right mindset is the most important thing you can ever have in life.
It, more than all other things, defines you and defines your actions. Your state of being determines how you treat others and how you treat yourself. You are not what people do to you, you are not how people treat you. You are how you treat other beings in this world. I think if people understood this they would have more self-confidence, but also be more inclined to act honorably because they want to be seen in a good light - so they focus on acting in a benevolent way rather than appeasing others to be treated benevolently and to stimulate one's vanity.
I'm not an Existentialist (technically, though I am an Existential Nihilist) but I do think this aspect of Existentialism has obvious merits as opposed to the alternative which I briefly described. Maybe you are a bunch of chemicals, maybe this world really exists. But we don't know that. If you're a thinking person then what you do know is that you have thoughts. And as a thinking being you make choices. Even if they are pre-determined (you don't have free will). So, define yourself by those conscious decisions, one of the only things you can be sure about in this uncertain world - uncertain in what happens in it and uncertain in its very existence. Be the person you want to be, and help others not because it'll make you appear good to others, or even because it'll make you good, but because you are good you know you should. On one hand we need to define ourselves by our actions rather than what happens to us; but on the other hand we shouldn't be worried to begin with about who we are but merely whose suffering. Leave questions of identity to the tabloids and Existentialists if you can. Not worrying about "being," just doing.