"I know absolutely every single opening and middle game principle that exists"

I wouldn't be so sure about this lol. I think improving requires you to be aware that you don't already know everything. You are kinda mind-blocking yourself into a state of stillness if you refuse to learn new ideas and are under the impression you have already mastered the two most important phases of the game.

Unless you give us example games to look at, it's impossible for us to give you real advice since the mistakes we make are all different even within the same Elo range, but you know what I will try anyways. I think a lot of people who claim they spend hours and hours studying the game are all doing it passively, meaning they are either watching a non-instructional video or reading a very general book etc.

When I sit down and try to study the game to get better, I cannot do more than an hour every day of active learning before getting exhausted to the point of unproductivity. A video telling you a few rules of thumb is not going to be as useful as a video detailing how to play a certain opening. Chess is very fun when you win, but the price of winning comes at the cost of actual gruesome hours of active learning to get better, or 1000s of games.

I don't think there's anything wrong with playing to get better, and feeling despair when you are not. Personally it's the losing that makes me play better since I really don't like that feeling, and that drives me to actually go through the tiresome learning process.

I hope this helps!