Chess engines have ruined openings...

...for the competitive players below FIDE titles, that is.

At the top level, engines have actually both increased the viability of some openings and decreased the viability of others; however, the main problem I've got at my own level (and - and I cannot stress this enough - my rapid Lichess rating, rather than any of my Chess.com ratings, is the truly indicative one, if you wanna judge me by my games) is the following: the more I try to optimise my opening repertoire so that it becomes something I can use in classical games as well without getting prep-nuked, the worse my results become.

I used to pour hours upon hours into analysing variations myself without consulting any engines, based off of what I thought were the most natural and/or logical responses my opponent could come up with; but alas, once I started using engines to check it, aside from outright blunders, the evaluational numbers rapidly started becoming more and more disheartening. I found myself trying to adopt opening lines that were correct, but since I had no learning naterials that would explain to me how and why they're good, I started half-assedly memorising them more and more, and I don't know of anything that's hurt my chess abilities more than that.

How come that striving for the more objective truth in chess is making me a weaker player? I want it to finally stop. I feel like my brain is rotting.

And yeah, this title is admittedly a bit clickbaity, but still, there's just something so wrong about wanting to play more objective chess and failing due to how apparently incomprehensible it is.

why dont you just play stuff you can play naturally and skip this problem altogether?

Honestly, that's just the thing - I'm not even sure what stuff I can play 'naturally' at all after do much engine junk consumption. It's like I've lost my feeling for all openings, more or less. Sure, I'm still more familiar with some of them compared to the others, but they just don't... speak to me anymore, y'know? I get move orders wrong, I don't know what thematic transformations to go for and when, and making decisions takes even more of my time and energy.

Lately, even when I do happen to win a game, I end up feeling like I got there through a confusing mess, by pure chance and with little to no understanding of what happened. I think I crave clarity more than victories.

There are many, many viable openings. But if you aim your repertoire at classical time formats you're just going to restrict yourself unnecessarily. Target your repertoire for rapid games.

There are usually many viable moves within a given position, as well. 
A rule of thumb I follow is I try to keep the engine eval better than +0.25 to +0.3 as black, and better than +0.00 for white. That's going by leela, though, who tends to score things closer to 0 than some other engines. In stockfish that'd be +0.3 to +0.45 as black. Which is about where something like the Scandinavian or the Pirc would score. Or if you're white, something like the Trompowsky. The Jobava is just below that but close enough and very dynamic. There are some exceptions - if an opening just gives tons of activity and in practice is scoring very well, like the Benko.
So within that "currency" leeway I give myself, the rest of position evaluation is all about the human factors. And there are many... you need to learn to evaluate positions. Start by understanding of opening principles, and chess principles in general. But you also need to refer to a database for the stats.... game frequency, winrates, at different elo levels and time formats. Lichess has one. The engine has a role too, but mainly in brainstorming options for moves, helping you to assess how sharp the position is (like if there's 1 good move - it may be hard to play. Some positions just cut against the player), and so on.
Engines haven't ruined the game, you just need to learn how to think about the game.

only up to 0.45 as black? half the fun stuff is at the 0.6-0.8 range!

What an evaluation doesn't tell you is how hard the game position is. I might have a +3 evaluation in my favour, but if it is only a single line requiring several difficult moves to play and calculate, with all other lines being bad for me...I'll probably lose.

you need to learn to stop giving a damn about the eval bar.

"how come striving for objective truth in chess is making me a weaker player"?

Because you are too weak of a player to strive for that goal.

It is like a third grader trying to understand calculus. He doesn't even know math has letters yet, let alone words. Limit? How do you spell that? What do you mean it goes infinitely close to zero? You try to explain it by having him imagine dividing something by two repeatedly, but you realize they have no concept of fractions. Or division, for that matter (varies by individual). And any time they try to apply what little they half-understand is almost certainly done wrong or in a useless manner. He rides his bike to school, nearly crashes into a wall, but steers away last second. "This is just like what I learned! I was really close to the wall but I didn't hit it."

Chess is also inherently not an objective game, although there are objective elements, like tablebases, for example.

Take art. Is it objective? I'm sure there are objective parts we can agree on. Perspective, for example. Anatomy for animals. But that doesn't mean two people will like the same piece of art. Perhaps one person really likes hyper realistic paintings, and another prefers eastern style paintings with simple brushstrokes. Maybe a third doesn't like paintings at all - she prefers sculptures. Paintings don't interest her.

In chess, one person might prefer to play hypermodern openings, and another player might like classical ones. It's not confined to openings, either. Maybe you might choose a safe route instead of a risky one, even if the latter is objectively better.

Let's say you had a choice between mating the opponent in 7 moves, or between winning a piece and playing on. You don't know it's mate in 7, nor can you visualize it that far, but you think you have a mating net. Do you go for the piece, or for mate, and risk losing it all? Different people will pick different choices. And these choices are very abundant in the game, and they all contribute to whether you win or lose.

Just because a computer says a move is better than another, doesn't mean that you have to believe it, because it may not be true for you.

Would you say d4 or a3 is a better first move?

Obviously, the correct answer is d4. It simply does everything - opens up a bishop, controls the center, takes space. The best argument you can make for a3 is that you can play as black with an extra a-pawn move added, which isn't a very convincing argument either.

But I score much better with a3 than I do with d4. In fact, I score better with a3 than any opening that I've played seriously. And even funnier, my score with d4 is only better than a completely troll move, d3, and b3 (which is a perfectly reasonable opening that I simply can't seem to play right)

Lastly, your performance could be bad because you simply do not play enough. You don't just learn an opening like you would change clothes - "I'm going to play the ruy loepz today, because the ruy is good!" - and expect to start winning.

"What do you do when black closes the position with c4?"

"You mean white?"

"No, black."

"What?"

Also, learning a new opening naturally comes bundled with losing. It's a buy one get one free deal. When I was learning the english opening, I lost around 200 points online and it took a month to climb back up. But now it's one of my most comfortable picks, and I try to transpose into english positions as much as possible.

This "losing" can be seen even at high level.

For example, GM Edvins Kengis tried to innovate a new line in the Alekhine's defense:

His first three games in this line were losses. But now if we check in on his line, it's clear he figured something out:

I second darkunorthdox ...

If your fixated on that, you probably fixated on searching and using a Theoretical Advantage from an opening line...

GM Noel Studer On Openings... Quote: Harsh truth No#1 " There is no perfect opening" Harsh truth no#2 "The More you know, the worse it get's"

https://www.youtube.com/watch?v=xDna9eIBM8M&t=441s

Some people think the engine eval is everything. Others just discount it entirely. The reality is somewhere in between there.

It took me a long time to decide what openings I liked. I played many. In the end, after alot of experimentation, I had a sense of how the different lines felt to play. Only then could I really make up my mind. I narrowed it down to a few... there are 1-2 that I still juggle but not many.

I think that sense of how an opening feels to play for you is very important to connect with. Because there are personal reasons I like one line over another. I can't stand playing the London. It's just grindy and boring, and not my style. I also can't stand anti-sicilians. From the lines I liked, I went with the lines I enjoyed the most, and which suited how I like to play.

After playing the Van Geet, 1. e4, 1, d4, and other lines... despite what anyone says, games were just noticeably easier when I played the Van Geet. Players hadn't practiced the line 1000 times, like they had after 1. e4. Even though I had an interesting, novelty-focused e4 repertoire, players still respond better to positions that vaguely correlate with what they usually play, structurally and in other ways. I've settled on the Van Geet and have remained content with it for quite a while now. Again, not a decision I could have made without having felt out what it was like to play the lines.

But I wouldn't have arrived at those lines if I didn't go through a rational process, which included engine evaluation as one factor for input, among many others. The Van Geet was close enough to being within my "leeway range" as far as the eval goes. The line that's strong against it is a very sharp one which no one ever plays. I almost never run into it. I could tell that from the stats. So I'm not someone who dismisses the engine categorically. That's really just foolish. You do need to learn to assess positions, which involves many things, but engine eval is a part of it. It will always be. 
Because it's true, we cannot objectively assess a position - and therefor, when we reach the limit of our ability to assess it, some external form of input is a good augment for decision making. And the engine eval does say something about the "gestalt" of the position - though you may not be able to define what that is. But that's why I use it as a brainstorming tool and often as a tiebreaker. Keeping the eval within some threshold also ensures that your repertoire is *sound*, i.e. if at some future point you want to buckle down and develop the repertoire more rigorously, and learn the theory more deeply, maybe for use in a classical context or against people who can prep against you, you wouldn't be wasting your time.