Assertiveness is behaving in one's own best interests, standing up for oneself without being anxious or guilty, expressing one's honest feelings comfortably, and exercising one's right without denying others theirs.
Practice assertiveness by being firm and demanding yet soft, direct and respectful.
MORE IDEAS FROM THE ARTICLE
Most people are either passive or aggressive. Passive people are afraid of confrontation and lie easily.
Aggressive people are not liked, as they can trample others for their own benefit.
The middle ground, assertiveness, is where you want to be: Respectful, firm, observant, and detached.
Worrying is the mental habit of trying to solve a problem that either can’t be solved or isn’t really a problem.
It gives us the illusion of control. Worrying about it won’t change things. But it will lead to a lot of anxiety.
Being factual about what you don't like in someone's behavior (without overdramatizing) is an important start.
The same should be done in describing the effects of their behavior. Don’t exaggerate, label or judge; just describe.