If America is a democracy, why is it so hard for us to stop laws that would negatively effect us and our lives?

Democracy is generally defined as a form of government in which all adult citizens have an equal say in the decisions that affect their lives. (Wikipedia) If this country is a democracy, why are we practically powerless to stop detrimental bills, amendments, and laws like SOPA and PIPA ?

EDIT: Okay, so America is not a democracy, I understand this now. Look at my username, I go to a subpar high school because of the assholes in our government and in charge of our large corporations. Thanks though.