If you doubt that positive thinking does work, consider the alternative question. Does negative thinking work?
To answer this for yourself consider this challenge.
Years ago when I was just starting my career as a teacher I wanted to test out two different teaching concepts. Praising and correcting. I decided to spend one full day where I would only correct the actions of my students. If they executed a technique I would tell them what was wrong with it and how to do it better. If they did something totally correct I would ignore it because I was only going to use correction. Can you guess what happened? First the performance was one of the least productive classes I had ever ran. Second, I felt like crap by the end of the first class (and wanted to kill the experiment right there). Keep in mind in doing this experiment I was trying my best to not have an angry or mean voice tone while I did this test. I tried to stay as neutral as possible while I told people why their performance was not up to par.
The next day I used only praise. Well, to be truthful I used only honest praise. When a student practiced I looked for something that was done well. If the technique was a little off but the effort was there I would say. "I can see you put your heart into this. I like your spirit." If the effort was low I would look for one detail that was done correctly. I would say, "See that, you tightened you first on impact. Nice work!" Now I had a plan for dealing with a student if they did the skill 100% wrong with 0% enthusiasm. If that were to occurs I would ignore that student and look for someone doing it well. However, I did not have to use that part of my strategy. Can you guess what happened? First, performance improved massively. Second, students were excited and had a good time. Third, I had one of the best days of work ever!
I find that when people claim, "That positive thinking stuff is a bunch of crap." They usually say that in a negative, unhappy tone. Proving that they are practicing negative thinking.
If you still have your doubts about the power of you mindset, take my challenge. Spend one full day where you only; correct, complain, blame, worry, gripe, condemn, reject, say no to your desires, argue with everyone, and get massively stressed about every single thing you do. Do not let yourself have even one positive thought the entire day. If you do catch yourself; smiling, appreciating, complimenting, laughing, or in any other way feeling good... Reprimand yourself immediately and get back on track. What do you think the result would be?