Something like four months ago I had this weird revelation that maybe all of ethics was bunk. I've had a couple moments like this in my life and most haven't stuck; I assumed this one also wouldn't. But, well, it's been a few months and I still think maybe ethics is fake. And weirdly I think it's made it easier for me to care about things, and do things that I think matter.
Why I changed my mind
I used to basically want two things from ethics. The first was to know what to do, because wow, the world is very confusing, and I often find myself in tricky situations where I really wish there was a good answer. Call this action-guiding. The second was to do good, to be a good person who did good things and cared about stuff. Call this goodness. I now believe neither of these is a good reason.
Basically I don't think any ethics is action-guiding. I spent a few years doing moral empiricism - actually trying to be a stoic, an existentialist, a daoist, a virtue ethicist, even very briefly a kantian - and they're all terrible at telling you what to do.
I came across the action guiding objection in the context of virtue ethics, and it's true: "be wise and temperate" really isn't an algorithm for making choices. But none of the others are better! I read the Ethics of Ambiguity and internalised the fact that it's actually pretty hard to abstract the morally relevant details from a situation. Yeah, once you have the maxim, you can run Kant's test on it at least sometimes, but pulling out the morally relevant details of a situation is hard!
Even consequentialism isn't really action-guiding, because utility calculations are debilitatingly hard. It would be a great moral system for someone omniscient, but our hardware is basically terrible for everything except virtue ethics.
So, crux 1: No ethical system is actually action-guiding.
(mumble mumble "what should we do" is not the same as "how should we decide what to do", yeah, sure, but unless we can solve the second one, the first is sort of irrelevant, and I have yet to see anyone get from A to B).
There's this argument that swirls around sometimes in various shapes. One version is this: "Whenever you make a choice, you are picking the thing you most want to do, so you are choosing based on your own desires, so the choice is fundamentally selfish. Even if you throw yourself in front of a train to save a stranger's baby, it is essentially still you doing the thing you want to do, so there is in fact no such thing as altruism".
I think this is just wrong.
The very brief explanation of why is "it seems genuinely simpler to describe people as actually caring about the outside world". Notably you don't lose any predictive power. On the other hand, the "secretly selfish" argument needs a whole bunch of epicycles. More specifically, I think most of the time people make these arguments, it's coming from some kind of place of confusion.
A longer and better version of this argument shows up in replacing guilt. Personally, I first encountered it at a 9am philosophy lecture in my first year of undergrad. I remember it because it's one of about four times in my whole degree where a philosophy tutor spent half an hour making an argument that genuinely permanently shifted my worldview. Big thumbs up.
Weirdly, though, the version I managed to cache was the weird mid-2010s internet atheist dialectic where the theist goes "but if there was no god, wouldn't people just start murdering each other on the streets?" and the atheist goes "what the heck are you talking about?"
So you don't need ethics to care about stuff and want to do good things. And ethics isn't actually going to help you work out what to do about the things you think matter. To the extent that it even could, you'd be treating it not as an ethical system but as a decision-making procedure.
And these are great! Virtue ethics is a pretty good heuristic for daily life; rule consequentialism is a reasonable heuristic for massive confusing decisions under heavy uncertaintly; stoicism is a great way to deal with bad feelings. But I'm pretty sure none of them are actually real.