If ancestors knew about it, why don't we apply it today? (+solution)

In 1600 ad. Shakespeare wrote a play "The Tragedy of Hamlet, Prince of Denmark" and inside it, between ingenious lines, it is one: "For there is nothing either good or bad, but thinking makes it so". At first sight it appeared to me a general truth ("duh, of course!") I wanted to skip it because I know it! but then I realized that people seldom practice it. Even I, in weak moments I lose control of my thoughts and make a catastrophic event out of nothing. However, I was thought to think positive (I am lucky with my mother) and to see the good part in every bad part and even so, my negative emotions get to me sometimes.

My question is: If somebody in 1600 ad. wrote this fundamental idea and many more since then talked about it, why people today don't practice it? 

There is a paradox here because I know the truth in this statement, however I cannot put it in practice all the time. For me it happens mostly related to high desires, when I really really want something and I am afraid of not having it. In low desire situations or when I realize my slip to the cloudy side I meta-think, I see my thoughts and I have power to change the direction.

But I am puzzled how ideas from 1600 ad. that are out there and people around me know about them, reach me later in life. These type of ideas are indispensable for a great living but are ignored, or difficult to put in practice. How should be find and apply them in our lives so we can live better?

I like the 'implementation intentions technique" it is a genius marketing tool to get customers buy from you. It implies that if individuals set an objective that has both an action and a time frame, they will do it in 67% of the time eg "I want to do X in Y weeks". Since I found out about this technique I use it all the time, amazingly it works!
But I would really want to find out other solutions; If you have any ideas, please do leave a comment.