Basically you are either a natural, or you are pushed for it.
Being a female and international/foreigner professional on a high-competition work line, I have seen too many injustice, unnecessary pressure, back stabbing, sexism, racism, nationalism, mansplanning, bias, discrimination, stereotyping, and all bunch of other shit.
That is the annoying reality, which I hope one day will be eliminated. We gotta do this.
Working with others in such an environment also means that they will try, at one point or the other, to impose these on you, and control you or your work.
I have had a recent experience just yesterday that pissed me off too much. And this morning email reply has been sent; sane, professional, and firm that I will not accept this because of this and that. I am proud of my self for doing this. My consciousness is quite clear. There may be negative consequences, but hey.
Some people will need a break to understand that they are crossing the line. Assertiveness helps you to convey this message to these people and help you keep your self-respect and protect your rights. I never knew how assertive I could be. I am still hugely annoyed by the behavior yesterday, I hate the feeling and experiencing this, but I am grateful for finding the right type of reaction after hours of agitation, fuming, and pain (that is assertiveness, not attacking back, complying, or passive aggression).
By the way ladies, this is a separate experience, but do you not love it when male co-workers cut your sentences? Both my superiors and team members have done this to me and hopefully now have learnt not to cross their lines. Mansplanning, is it?
I need no this or other shit.