The more positively you think, the better you'll be, the more negatively you think, the worse you'll be.
Our brains are made of “circuits.” These circuits change daily/overnight. We can/do rewire our brains, intentionally or unintentionally.
This is done by thinking. Any thought you have. Good, bad, in the middle, whatever. We can use some common sense here. More good thoughts = more good, more bad thoughts = more bad.
So, if you think more negatively than positively, you're creating new “negative/bad” connections. If you think more positively, you create more “positive/good” connections.
This is why “positive affirmations” can work. Telling yourself good things over and over can eventually create more positive connections in your brain, and you'll have a better outlook on life.
If you constantly tell yourself bad things, the opposite will happen.
I am partially an example of the latter. Due to my anxiety, I would always question myself, even when others tell me how great I am or all the good things I do. But, I thought otherwise (mainly from the few who talked shit, or tried to put me down, and make up things), and it got to the point where I started really believing my negative thoughts, and I actually started becoming the very thing I thought I was….but after realizing that, I stopped myself, and am a more positive person. I am not the negative person I thought I was, an avoided unintentionally becoming that.