3 simple steps, $60 and hundreds of hours of work. (It’s like choosing a diet — the ones that require the least exercise are the least effective.)
1. Take the scoresheets of your last 30 rated games. Pick out the opening you dreaded most. Buy a book and start playing that opening WITH BOTH COLORS. Yes, even if you don’t play 1.d4 or whatever. Do it anyway. Study this opening.
When I say study, I mean:
a) Choose lines, play them out on a physical board, and write them down in a notebook until you can reproduce them from memory, with the resulting positions clearly visualized in your mind.
b) For each line, write out a positional goal or set of themes.
c) Ask questions and answer them in writing.
Example: In Play the King’s Indian, there are lots of lines where Gallagher says “White would like to tempt Black into playing h6 to weaken the king and prevent the Bg7 from being activated through that square.” Then there are lots of other lines where he says “…h6 is a good idea in this position. Okay Mr. Grandmaster, why doesn’t the previously stated objection apply in those positions? I wrote that question in a notebook, wrote out lines where he says …h6 is good, and studied them until I could articulate why.
2.Pick out the middlegame where you felt most uncertain, or where your opponent demonstrated to you that your ideas were completely inadequate. Study that middlegame and steer ALL your OTB and blitz games toward that exact type of position. Buy a book and study that.
That means if you score well in tactical melees, you’re going to start trading queens and playing symmetrical pawn structures. If you are happiest playing fluid structures like the Pirc or hedgehog, you’re going to start playing fixed centers. If you avoid trades, you’re going to start simplifying early.
3. Scan all the endgames and choose the type where you scored the worst. Rook endgames? B v N? Simple ones, complicated ones? Study them. Buy a book and study that.
Ignore your rating for one year and do this work. Learn to enjoy these positions. At the end your playing strength will be 100 points higher.