Being assertive means standing up for yourself without losing sight of the feelings and interests of another. Assertiveness is about setting boundaries. But in practice it is not so easy to be assertive. After all, we don't want to put pressure on the relationship with the other person or we are afraid of the consequences if we step on our toes. So emotions often come into play.
Yet you can learn to stand up for yourself in an assertiveness training.
Read articles and tips on assertiveness here