Why is my chess game slipping?

First, stop blaming your age. 46 is not old.

Second, stop. Don't just keep on keeping on. Don't expect a different result if you keep trying the same thing over and over. Take a different approach. Here are three alternatives:

Alternative 1.

Get a coach with rating over 2200.

or

Alternative 2.

Study Artur Yusupov's 9 book training course that begins with Build Up Your Chess- The Fundamentals 1 from Quality Chess. Here is a description of the course and the other books.

http://www.qualitychess.co.uk/docs/14/artur_yusupo...

The 9th book will be released at the end of this month. January 2013. Series 1 begins with books for players about your strength and stronger. Don't be misled by the subtitle of Series 1, "The Fundamentals." It will be challenging. Don't skip to the book 2 series. Each chapter is followed by a quiz designed to disclose your weaknesses. Books for further study are also suggested. by Yusupov, and you might work in some of the books mentioned below in Alternative 3.

or

Alternative 3.

Your answer may be found in clues within your question. You have been studying additional openings but they have not helped. You are losing to simple combinations and in endgames. Doesn't this suggest the answer?

Considering your description of the situation you should be studying tactics and endgames instead of openings. Especially considering that this is the advice given most often to Class C players by every notable chess teacher on the planet!

Try:

Nunn "Learn Chess Tactics"

Weteschnik "Chess Tactics from Scratch, Understanding Chess Tactics" 2nd edition

Neishtadt "Improve Your Chess Tactics"

Neiman "Invisible Chess Moves"

Cheng "Practical Chess Exercises"

Alburt- "Winning Chess Endgames-Just the Facts"

McDonald- "Practical Endgame Play"

Schiller "639 Essential Endgame Positions"

or if your endgame is seriously deficient, start with Silman's Complete Endgame Course.

Take your pick of alternatives. I hope one works out for you.

You must study. Even top level Grand Masters still study. It's a must for keeping your skills sharp. I'd get Jeremy Silman's Reassess your chess 4th edition. I'd try Dan heisman's 20 minute chess exercise.

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If i only play and don't study my skills do the same thing as yours. They begin to dull. When I study, I am a monster. I have been beating 1700-2000+ lately. When I don't study, I will lose to a 1500 or 1600, it's horrible.

I'd recommend some of the mental exercise games on lumosity.com. They won't exercise your chess skills specifically, but if you feel that your mind is just getting sluggish, this kind of training could be helpful.