Part One of a Million Part Series, listed in no particular order.

You are not as good as you think you are, leaving you to make -EV decisions that you incorrectly perceive conversely.

You play against better rather than worse players.

You play too many hands.

You give tells that you are unaware of but do not read the tells your opponents put out.

You do not put in enough hours to overcome variance.

You are results oriented.

You act too quickly.

You go on tilt, subtly that you do not notice and just plain so.

You play too many hands out of position, especially as the opener in EP.

You are not willing to play significant hours one or two levels beneath your own perception of your talent.

You are possibly not playing the correct game for your skillset.

You do not play online.

You do not correctly use aggression on the turn in position when you think you are ahead.

You slowplay on the flop out of position.

You make but do not follow through on your reads.

You do not lay down on the turn to aggression on a selective basis often enough.

You do not put into action the thoughts and strategies developed away from the table.