The basic difference between being assertive and being aggressive is how our words and behaviour affect the rights and well being of others. — Sharon Anthony Bower
Assertively stick up for yourself. This level of personal initiative extends to people and organizations with which you are associated. Of course, needs are prioritized and have to be met. Interests need to be pursued. Personal and organizational goals need to be realized. For you, though, meeting your needs at the uncompensated expense of other people is unacceptable. As Euripides observed, Joint undertakings stand a better chance when they benefit both sides.
Pursuing self-interest in ways that prevent others from pursuing their interests is problematic. Reaching goals in ways that make it impossible for other people to reach their goals is to be avoided whenever possible. Sticking up for one’s self and one’s interests is consistent with your approach to everything, so long as every reasonable effort is made to avoid injuring other people and their interests.