Politeness and fair natured shouldn’t be perceived as a weakness, you don’t have to be rude and loud and to assertive. This article by Kim Tasso studies the difference between passive and aggressive behaviour, and imparts valuable techniques for being more assertive.
Everyone has some basic rights – to be treated with respect and to be able to express your views and to have your needs met. You also have the right to request more information if you don’t understand something. You have the right to change your mind and even to make mistakes. More importantly perhaps, you also have the right to decline responsibility for other people’s problems.