Question of Quest

My rating fluctuates. Sometimes I play great and at other times I wonder who I am. My Quest is to attain a higher rating. A rating I believe I am capable of achieving. Quest: four (4) years of chess and my rating is 1200 (1300 highest), what is the missing key(s)? How can I determine how to improve? Is there a standard set of theories I need to know about ? Can anyone point me in the right direction. At times I want to quit playing, but I can't-I love this game. Strategy, tactics, deception, sacrifice, gambit, etc, etc,.  Quest: any ideas!