Being assertive is different from being passive, aggressive, and passive-aggressive. You're standing up for yourself and your rights. Try the DEAR technique from Dialectical Behavior Therapy (DBT).
-D stands for Describe. Describe the facts of the situation. For example: "I was standing in line and you cut ahead."
-E stands for Express. Express your feelings using "I statements" such as I felt _frustrated__ when _you cut ahead while I had been patiently waiting in line ____.
-A stands for Assert. Assert what you want or say No clearly. From the prior example: Will you please give me my spot back in line?
-R stands for Reinforce. Reinforce the benefits of the new action. "I would appreciate the respect of having my spot back."
Listen to this quick podcast from the Savvy Psychologist Dr. Jade Wu for 2 great examples in daily life of choices we make in difficult situations.