Self-confidence (small translation)

A frist step to change your life is to trust yourself.

The first step to changing your life is to trust yourself.

The rest of the translation reads like this is the most important and first step you must take rather than any one of a number of other steps.

If you don’t believe in yourself, who else can? In order to become successful in your life, you have to learn to become self-confident. And to do that you have to choose to do that it! To believe in yourself you have to defeat insecurity, you have to be proud and sure of your possibilities, and you have to be optimist.

Generally speaking, in British English, lists of three or more items are usually in the form: a, b, c, d and e. In America: a, b, c, d, and e. Although the American style is preferred in certain quarters in Britain, notably at the Oxford University Press, and is called the Oxford comma by British Editors. In British English the comma only precedes "and" when the word occurs again in one of the items being listed or some other ambiguity that is eliminated by inserting the comma.

There are a lot of "… you have to …" fragments in there. Nothing really wrong with them but it becomes cumbersome to read after a while. An alternative that scans a little more easily is "… you must …". For example "To believe in yourself you must defeat insecurity".

Sure Of course, if you think “I can’t do that”, “I’m not able to do that”, or “I’m afraid”, you will hardly become successful in you tasks.

"in your tasks" is a bit superfluous.

Face challenges with determination and firm belief!

Conviction may be a better word.

Keep telling yourself “If I want to, I can make it!”.

Not sure this accurately captures the original meaning.

When you have a bad day (who doesn’t?) when everything seems to be going in the wrong direction, just think about nice things, take a deep breath and start again full of enthusiasm. In this way you will see that everything around you changes, but in fact it’s just you that that you are viewing them differently. 

To believe in ourselves also means to take on our own responsibilities, responsibility for ourselves; to set goals, and to do everything possible to achieve them. The more you reach the results goals you have previously set, the more you become self-confident, and that’s very evident!

You could also "achieve the objectives". Not sure evident is the right word. Do you mean obvious?

Face your problems as if once solved, they will give you an added value and let you better know yourself as a person.

Face your problems as though, once solved, they will empower you and help you understand yourself more.

Cancel Forget failures: every bad experience teaches you how not to fail again. Negative emotions are stimuli taking you towards the finish line.

“And what’s if I’m wrong?” Be wrong! And don’t be afraid of that. Being wrong is important because it helps you learning. The important thing is to believe in your choices and to immediately understand when you make a mistake. You will learn and then start again until it becomes natural not to make the same mistakes again.