I broke through a plateau by studying openings more than memorizing them. Slowly working through the early possibilities to separate what works from what doesn't helped to reinforce basic principles.
So like hammering away at 30+ identical problems in high school math class, with some tedium it really did help me internalize the principles. I did not continue to memorize openings and common lines to any great depth. I remain a meager, but much more contented, chess student.
An alternative approach is to work through endgame problems. Learning the key different endgames helps you direct the middlegame towards positions you know you can win. There's also something about learning to handle small numbers of pieces well which translates really well to the rest of the game.
