I believe that the reason is that we fall into the trap of focusing on getting new information all of the time instead of learning how to use what we already know effectively. I will use myself as an example. I have a chess library that, at its peak, numbered close to 800 volumes. At the risk of sounding immodest I believe that my knowledge of chess history and culture is at a high level. In addition I am well versed in most modern opening systems and have a solid knowledge of the classical players. In spite of this, I have lost hundreds of games by making very large crude blunders. Often I have given away my queen for nothing. Why does this happen? I clearly know enough to realize that I can't win games by giving away my most powerful piece for free. Why does this happen to me when I know so much about chess?
I think it happens because I don't effectively use the information that I have in game situations. I need to improve my thought process in practical situations when the clock is ticking and the person on the other side of the board has bad intentions. The key isn't learning new things all of the time but instead learning how to use what I already know effectively.
Now as much as I would like to take credit for this insight, alas, I cannot. Grandmster Igor Smirnov of Ukraine is an experienced chess teacher who preaches the point of avoiding "useless knowledge" constantly. GM Smirnov has a series of both free and commercial products that I have found very helpful in actually improving my play. You can check out GM Smirnov's blog here. His youtube page has a ton of free videos that you can see here.
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Next post we will get into the details of the thought process that I am trying to use and talk about my early results. To close for now, here's a sample of GM Smirnov's chess lessons: enjoy: