CroMagna,
I would recommend first that you learn how to defend yourself physically then mentally. As you learn to be more aggressive and confident physically, it will translate over to other parts of your life. You could join a good kickboxing or Muay Thai (kickboxing from Thailand) gym and learn how to kick butt.
Also, every time you have an experience that didn't go as you would have liked, you can visualize in your head how you would have liked it to go. After some time you will start developing better ways of handling difficult people as you will have rehearsed it so many times in your mind.
Lastly, there are A LOT of good books on assertiveness that you can pick up at your local Barnes and Noble. They will walk you through the steps to becoming more confident and assertive.
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