Assertiveness comes from within. You have to value what you have to say before you can put it out there, and you have to have confidence in yourself before you can assert yourself. Do you have self-esteem issues?
Even if the open windows of science at first make us shiver after the cozy indoor warmth of traditional humanizing myths, in the end the fresh air brings vigor, and the great spaces have a splendor of their own - Bertrand Russell