There are great resources and books about assertiveness out there. Just like listening and validation, at first it may feel like reading scripts robotically. Over time, it just comes naturally.
Just like detachment, I think there is "loving assertiveness" (ha!). It's really hard having those performance reviews at work, I've been there. It can feel like a really negative thing, giving someone a warning, but really what you are doing is giving them honest feedback so they can hopefully improve and have opportunities for professional growth. It is a kind gesture (although they may not receive it as such!).
