Whether you are playing White or Black there is almost always at least 2 plans of attack and 2 defensive systems. You need to read and study about these plans in a book. The Everyman Series of books on the opening explains in words the plans of attack and defense for the particular opening. Every opening results in one of 6 characteristic pawn structures. There is a book by Silman titled Pawn Structure Chess that covers each of those 6 and strategies and plans that are involved with each.
Tips for the middlegame?
I simple middle-game strategy that i've been using when I get stuck and don't know how to improve my position, is to start pushing a pawn toward my opponents castled king. In this game, at move 8 I wasn't sure how to develop my pieces more, so I began pushing my h pawn to weaken my opponent's king's pawn wall.
You just need more experience; keep playing games, slower preferred. That's the only way to conquer your nerves and develop an at-the-board composure, so to speak. Confidence is key as well. I've heard some players say that hating your opponent is a successful way to keep psychologically stable during a game, and it makes sense. Anand said that he doesn't hate his opponents during games but that this mindset is common, even among the elite.
I'm completely fine at the opening. When white, I make a good opening, when black, a good defense. But when there comes a pawn break/threat/capture, and the game goes into the middlegame, I lose confidence and become worried that my opponent will overpower me in all of his moves.
How do I get rid of this? It's basically the reason I play badly and lose a lot.
If I wasn't nervous and jittery, maybe I could have won a lot more games.