While I believe that changing what you think about yourself can change your life, a few notes:
Positive thinking is not a substitute for medical or psychiatric help in actual mental health issues. A lot of deeper problems are better served by one to one therapy/counselling.
Just believing you can do something difficult or dangerous is not enough to do it. I could believe I could walk across Niagara Falls on a tightrope, but I would die. It would take a lot of work and practice, and the self-belief can help you out the work in, but it can’t take the place of putting in the time. Don’t do anything silly.
I can’t guarantee any specific result, and you can’t change what happens to you or other people’s choices. And this is not a way to think yourself rich. You might be able to get yourself into a mindset that allows you to achieve more, and then get rich that way, but this is up to you! And my bank details are…
This is not motivational positive thinking fluff. Life is real, and being positive won’t make bad things go away. It will help you to deal with them better though.