Existentialists are nihilists because they recognize that life is ultimately absurd and full of terrible, inescapable truths. They are anti-nihilists because they recognize that life does in fact have a meaning: the meaning each person chooses to give his or her own existence. They recognize that each person is free to create themselves and make something worthwhile of themselves by striving against life's difficulties.
Existentialism holds that you can only truly change the way you think and feel about your life by behaving differently, by acting rather than simply reacting, by asserting your will rather than simply allowing yourself to be swept along by circumstances, by always taking responsibility for yourself and what you do. - Gary Cox