The good news is the answer is easy: You don't study enough, you don't post enough hands, you don't discuss poker enough with players better than you, and the list goes on. Try coming into the #flopturnriver IRC channel to talk with the 20-30 people who are in there all the time about poker.
The bad news is (because you're a human being) there's a high chance you won't allow yourself to get better because it would require you to realize you're not nearly as good as you think you are, and because it will take some serious work to improve.
In any event, good luck.
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