What makes me a poor player?

Play long games, ideally OTB tournament games where the time control is at least 60 minutes each side. Try to use all your time.

You'd be surprised how much that helps.

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Ok, maybe you wont do this, so I'll give you some exercises you can do solo that will simulate the same kind of work.

Get a tactic book (not the online crap) and use some puzzles that are a little too hard for you to normally solve.

Then stare at the position and calculate, trying to find the solution, for a long time. 10, 20, 30 minutes is fine. Write down your main variation and all sub variations then look up the solution.

This is not a tactic exercise, this is a calculation /  analysis exercise.

You can also do this with GM games, guess-the-move style. Spend at least a few minutes on every move (yes even simple recaptures) and write down your analysis.

By the way, writing down your analysis should not be used as an aid. Calculate everything in your head, and only after you're completely done with a line write it down.

Do this 2 hours a day, every day, for a month, and you'll notice a definite improvement in your calculation accuracy, blunder checking, stuff like that.

Thank you!