Assertiveness, some things you may not know

The assertiveness it´s commonly defined as a learnable skill, the skill of saying whatever being confident enough to defend your rights or convictions, always in a respectful way. It belongs to the psychological field, it´s located between the two polar behaviours: aggressiveness or passiveness.
The assertiveness, as any skill, can be trained, and should be trained, it´s going to help everyone, not only those who always feel guilty, also those who don´t have much confidence and are not use to express their beliefs.
An assertive person accepts his mistakes, says what he thinks without caring about what other people think about him, and it´s able at the same time to defend what he says.