I empathize with Your Plight, sagremus. Quiet, Even-Keeled People are often seen by Bullies to be Fodder for Fanforonade, and you've recieved Some Good Advice and Perspectives from the Other Posters.
I have no trouble being Assertive and Calmly and Objectively Confronting anyone on their Inappropriate Behavior. However, My Lady Belinda tends to be more like yourself, sagremus. She worked with a Doctors Group for 17 years before going into Med/Surg Floor Nursing. She flowed with her Previous Place of Employment but was being somewhat Bullied by Some of the Staff in the Hospital Unit.
I worked with her for Quite a While before she could be Assertive, yet be Herself, when Confronting Inappropriate Behavior from Supervisors, Peers, and Co-Workers.
Belinda and I would discuss a Particular Scenerio, examine the OP's Behavior, Her Thoughts and Feelings on the Matter, and Brainstorm Ideas for Different approaches.
In One Instance, when Belinda knew she was going to be Called on the Carpet by her Supervisor for an Incident she was present for on her Unit. We practised some Role Playing, with me Playing the Part of Her Supervisor. Being somewhat familar with Belinda's Supervisor's Behavior, I approached Belinda in a Similar Manner that I believe Her Supervisor would. Belinda and I worked on Her Answers.
For example, I encouraged Belinda not to start a Statement with "I feel as though..." since Feelings are based on Emotions and Beliefs are based on Facts. If something is A Fact, a Belief, then state Your Belief as though it were A Fact. So, instead of saying something like, "I feel as though the Nurse did what she could in this Situation", Belinda rephrased her statement to say something like, "The Nurse acted according to Policy and Procedure in dealing with this Situation".
Belinda and I worked on Different Questions and Possible Retorts in Our Role Playing Session, almost like a Lawyer and a Client. By the time Belinda had her meeting with her Supervisor, her Anxiety had decreased, she felt more Self-Assurred, and went through the Interrogation Process well.
Some of Us are Naturals when it comes to Difficult Interactions. Most of Us are Uncomfortable with any Sort of Conflict. Until I learned a Few Techniques and actually put Those Techniques into Practise, I, too, was uncomfortable with Difficult Interactions. Now, after dealing with Aberrant Behaviors for the Majority of My Life and Creer, I feel much more comfortable. It took Practise, Trial and Error, Studying People and Their Behaviors, and finally, knowing that I was Correct in My Judgement before I could OBJECTIVELY approach an Individual and Confront Them on Their Inappropriate Behavior.
Now, I don't Go Looking for a Fight and I Prioritize where I will Expend My Energies. Some Circumstances ARE best Left Alone. However, if Another's Behavior can Tick Off a Saint, then That Behavior needs to be deal with. And the Culprit needs to HAVE to deal with The Natural Ramifications of Their Actions.
The Very Best to You, sagremus. Good Luck in Growing a Thick Skin without loosing any of Your Sensitivities. We need People like you as a Point of Reference for the Rest of Us.