Assertiveness is hard if you don't understand how it is different from aggression, what it does, or why it is important. Assertiveness means standing up for yourself and the things you believe in without attacking or putting another person down. It is a skill that requires self control, personal strength and clarity on what you believe. Assertiveness never involves behaviors like shouting, insulting, threatening or swearing at another.
It is important understand assertiveness is something you do for yourself to let go of negative emotions. It is not about getting your way or setting the other person straight, as much as it is about saying your piece and feeling good about yourself in the process.
The reason this is important, is if you think assertiveness is about changing the behavior of the other person and that person doesn't change, you can feel as if assertiveness has failed you. Anytime we try to control other people's behavior we set ourselves up for potential failure.
Assertiveness is done to build yourself up and let other people know how you feel. It is a way of venting emotions that would otherwise bottle up inside of you and eventually surface as an emotional explosion or illness. The trick to being successful at it is to stay calm, make your point and then let it go. If you continue to rethink the scene in a negative way, you will not reap the benefits of standing up for yourself.
Although changing other people's behavior is not the goal of this act, one of the great things about being assertive is that an ill-meaning offender will be counting on you reacting passively (doing nothing) or aggressively (attacking). When you assertively respond with strength and composure it throws him off balance which can result in a change in his behavior. Sometimes this will create a reluctant form of respect, causing him to change his behavior towards you permanently. When this happens consider it a bonus as you reap the rewards of your emotional vent and witness a positive result from your actions at the same time.
If you are already good at being assertive keep up the great work and be sure to model it for others (especially children). If however, like many people, you have gotten assertiveness confused with aggression, or have been playing the victim while others walk all over you, it would be a good idea to start working on developing this assertiveness skill. Assertiveness grows from confidence and confidence feeds off being assertive. Sounds like a worthwhile investment to me.