What is the best thing to study in chess to improve fast?

Recently I've hit a wall with my chess and realized I've come to the point where I must begin to start studying chess to improve. I've been going strong for about 5 days studying around 7-8 hours daily, but instead of improving my rating has actually decreased. I tried reading 2 middle gamebooks, but I don't understand really how to learn from them. I truly don't even understand what I should be focusing on to improve my game, and I've actually become scared of playing rated games. Any suggestions from higher-rated players on what you studied to increase the most when at my level, and how to gain more confidence when playing chess. 

don't go play rated games, start with the non rated ones against ur friends... or spar with engines...lower the engine's strength and adjust it with yours

and i would rather put more focus on studying endgame than midgame. as for opening, don't bother with anti mainstream ones.

as for confidence, i can't say any words since i also have issues with my (over) confidence sometimes

Take it from me, you are probably overthinking the matter.

7-8 hours is a lot of time to spend on chess! You probably could spend only 2-3 hours and get the same results(whether increasing or decreasing). As a recommendation, I would suggest to just cool off for a minute, regain your thoughts and then play when you feel comfortable.

There is no need to rush yourself because you are not playing in an OTB tournament. This is just online play, so you can relax and not put so much on the line. 

Again, better it to enjoy, and of course we want to improve, but we first need to seek the satisfaction and then improvement will come.(not saying you are not enjoying the game)

Now... As for the improvement. If you want to get better, I would suggest analyzing your games, consistently(if you don't already do so). Check which type of errors you make the most. eg. phycological error(pressure), 1-move blunder(but for your level that would be very rare), lost in calculation, tactical/positional oversight, external distractions(or your environment).

The possibilities are nearly endless, however, you need to understand how you lose, to know how you can win. Think about it... Most of the time, it is not that people play brilliant chess(referring to amateur's), but rather we outlast our opponents positions, and use certain chess skills like initiative, positional play, tactics, endgame principles to gain advantages, that our opponents allow us to gain because of the slightest inaccuracies, or the biggest blunders. 

In this case, I would just say, don't force a game to win. That's what I can say. But in truth, it depends on your style, and knowing how you lose, can help you learn how to win in the future. Again, I know it sounds very generic, but if you take this part, step by step, then you can understand how you can become better in analyzing, evaluating and controlling your positions in your games and therefore improve your skill and rating. 

Hopefully this was helpful.

Study is not the main thing.
The main thing is play - lose - analyse. Learn from your losses.
AlphaZero used this alone to reach 3000 level.
Also train tactics. Solve some tactics puzzles as a warm-up before you play. Chess is 99% tactics.
Study endgames, especially rook endings as these occur most often.
Study annotated grandmaster games. Learn from the best.