Unsure of what to study or how to study

I am rated around 1600, though I have fluctuated tremendously between 1500 and 1700. I am unsure as to how I can improve. I have heard to study games, but which games?

I have tried a few things:
1. Tried to look at all of my games in the last 7 days and try to figure out what the big swings were that caused me to lose. I tried to classify these, such as "left piece hanging" or "opening misplay", but I found that after just a dozen games or so, I was getting fatigued. I am unsure as to what the value of this is, if at all.
2. I have tried looking at insights, but it seems like a moving target. I identify one opening that I need help with, only to encounter many other openings for which I am unprepared.

3. Tried settling on a few openings and then learn as many of the variations as I can. This results in the same problem as the previous ones -- fatigue, and the feeling that there are too many to learn.

I am getting frustrated; how do people study for this? What do they study? Is it all just memorization?

study the middlegame its where the most amount of blunders happen

And how do you do that exactly?