"I, therefore, find it pretty impossible to justify my political positions to myself because they are probably expressions of self-interest"

Of course they are. And politic is a misfortunate thing in itself.

So I should be like Epicurus in regard to politics?

I find your post a bit confusing. If you are a ethical nihilist (I'm taking this as you saying there is no right and wrong) why do you think it is problem to justify your political position? Of course you can't justify it from an objective, rational point of view since you don't believe there exist one. On the other hand there is no way to dismiss it either.
In discussions you could say stuff like "from an utilitarians perspective..." or "according to Kant's...". So you take different positions and lie out the consequences of them. But I guess that won't be enough since you seem to be searching for an actual guide on how to live and make "genuine" moral and political statements.

I think it is a mistake to think that just because we can explain the historical development of (some of) our feelings we can reduce them to the evolutionary function they have had. Just because in evolutionary history it might have been in the evolutionary interest of the individual to be altruistic doesn't mean someone can't be a genuine altruist. To see if an action is altruistic we need to look at the motivation of the person, and not how this motivation has historically evolved. (I guess a comparison with the sex drive can be made. Evolution has developed the sex drive to make us create offspring. But when we get horny we often doesn't want to create babies, just have sex.)
Maybe I'm reading more into your post than what you are claiming here, but I felt I needed to get that out of the way.

So to my most constructive part of the post. Spinoza might be a philosopher to your liking and especially his Ethics. Nietzsche did like Spinoza.* Spinoza doesn't reject good and bad (normative judgements), but he does define good as "that which we certainly know to be useful to us" (Ethics, IV Definition 1). In short what is good for us is expanding our own power by gaining knowledge of the world which will make it possible for us to be as much as the cause of our actions as possible (as opposed to just reacting to outside forces), but this is enhanced by others also gaining power/knowledge which leads to a situation were it is beneficial/ethical for a person to help enhance the power of other people. So Spinoza is creating an ethics built upon the enlightened self-interest of the individual.

Ps: Was quite tired when I wrote this. Hopefully it is quite coherent.

* Quote from Nietzsche: “I am utterly amazed, utterly enchanted! I have a precursor, and what a precursor! I hardly knew Spinoza: that I should have turned to him just now, was inspired by “instinct.” Not only is his overtendency like mine—namely to make all knowledge the most powerful affect—but in five main points of his doctrine I recognize myself; this most unusual and loneliest thinker is closest to me precisely in these matters: he denies the freedom of the will, teleology, the moral world-order, the unegoistic, and evil. Even though the divergencies are admittedly tremendous, they are due more to the difference in time, culture, and science. In summa: my lonesomeness, which, as on very high mountains, often made it hard for me to breathe and make my blood rush out, is now at least a twosomeness. Strange."

I, therefore, find it pretty impossible to justify my political positions to myself because they are probably expressions of self-interest. — Particle thing

Why?

Error theory is an extremely implausible hypothesis. Are you absolutely committed to the thesis that the wanton torture of innocents is amoral?

So do people who subscribe to nihilism need some kind of working system of provisonal morality to navigate everyday life ? Even though that seems like doublethink. — Particle thing

Generally speaking error theorists simply dislike what we consider to be immoral, or like what we consider to be moral. Or they adopt a fictionalist account of morality, in which we act as if morality was objective and real, but secretly believe it to be a sham, in the interests of the well-being of whatever society we live in (which, presumably, we find to be morally important...? There's a real tension here in error theory, the motivation for adopting a fictionalist account of morality sounds suspiciously moral).

"This above all to thine oneself be true.'

No need to look to others to be yourself.. Probably lots of people will react in different ways and from these experiences you learn and evolve. It's the only way. To pretend to "take a rational position" gets you nowhere in life. Those who do pretend just founder and learn nothing other than it takes lots of energy to pretend.

Be yourself and change if you choose to.

I, therefore, find it pretty impossible to justify my political positions to myself because they are probably expressions of self-interest. — Particle thing

Bertie Russell says that if every person pursued enlightened self interest, the world would completely change for the better.

If nihilism bothers you, you're not actually nihilistic. If you really were nihilistic, then it couldn't possibly matter.

@Wayfarer

Although i would suggest that people often blurr Moral Nihilism with Nihilism. It is possible to be a moral nihilist withput bein nihilist.

I would suggest to @Particle thing that he is probably bothered by his nihilism due to a hangover of societal values, it is hard to embrace nihilim when everyone around you, in society, demands that you have an ethical stance on all matters.

Error theory is an extremely implausible hypothesis. Are you absolutely committed to the thesis that the wanton torture of innocents is amoral? — darthbarracuda

Why is it implausible?
Amoral is a slippery word to me. I would rather say it is false that torturing of innocents (whatever that is) is wrong (or right).
I don't think the difference between a moral error theorist and a non-cognitivist is that big. Moral error theorists claims moral utterances is about objective moral facts which simply doesn't correspond to anything in reality. Non-cognitivists say we are not talking about objective moral facts when we utter moral sentences, we are simply showing our approval or rejection for something. But since, for the non-cognitivist, this is what moral languange is they can still claim to be firm believers in morality.

I agree it is kind of a problem for error theorists coming up with a theory on how you should behave. The most plausible ones seem to ground themselves on some variant of what is good for the individual (some kind of enlighted egoism). But this of course assumes a person remains the same over time which is not obvious to me.

Although i would suggest that people often blurr Moral Nihilism with Nihilism. It is possible to be a moral nihilist withput bein nihilist. — Gotterdammerung

What is the difference between nihilism and moral nihilism?

Particle Thing: I wonder if you might find some comfort in acknowledging a distinction between absolute and objective morals. I consider myself a moral nihilist re absolute morals. But I accept the existence of objective morals just as I accept the existence of the rules of chess. It is an objective (but not absolute) truth that pawns cannot move backwards. I am comfortable abiding by that rule and taking it seriously when I play chess, even though I know it is not an absolute truth.

And anyone who asks "but is it really true that pawns cannot move backwards?" shoulders a very heavy burden of showing what "really true" means.

Same for the rules about not torturing children.

IMHO (at this point in my journey anyway :-).

Check out Spinoza's Ethics and Joshua Dienstag's Pessimism: Philosophy, Ethic, Spirit

First, I largely agree with your vision of the urge to truth. Truth is a god we want our side, and not always because we're just so wonderfully curious. If truth is not a tool or an idol, it's hard to see what it's good for if not to satisfy curiosity (which is to say as entertainment.) Is it the truth that truth is a tool? If "truth is a tool" is a truth, then what is it (as a tool) good for? It's a higher-level tool that allows us to use lower tools with more creativity and fluidity. If I "know" that knowledge for its own sake is not "really" (or no longer) my goal, then I can more efficiently seek knowledge as potential practice or prestige. I'm not stumbling around in the dark of a sentimental rhetoric of holy truth, a new god for the modern scientific man. Maybe I just want what I want, and the Mr. Truth pose is the long way around or a dead-end.

On the virtue issue, maybe being willing to live in this vision of truth is itself a virtue. I personally think so. I see the philosopher as a daring mind entertaining "threatening" ideas with a hard-won serenity. He or she lets go of one sacred "prejudice" after another. This exfoliation of prejudices is, in my view, made possible by doubling down a particular prejudice, which is that the "godless" or "fearless" critical mind is "axiomatically" noble and beautiful. That's the leap of madness that "founds" or makes possible the exfoliation of every secondary or non-essential prejudice, dogma. There's a traditional image of rationality that is, in my view, one more prejudice. The philosopher "transcends" (holds loosely) to various conceptions of rationality or frameworks for validating claims. He or she is constantly trying to get outside of every cage. Personality is a cage. The philosopher is perhaps an impersonality or anti-personality. That is one possible notion of virtue. (Our philosopher might be a productive citizen, loving family man, etc. A free mind doesn't necessitate a monstrous lifestyle. We can vote our interest.)

It seems like the rational and emotional parts of me are very much conflicted. — Particle thing

I would say you just proved the innate need to act ethically, and an inability to act unethically in terms of one's own DNA-commanded ethicality; by declaring that you have reasoned to not consider ethics in your day-to-day operations, yet it does not feel good to do that (emotions speak against behaving unethically).

This alone should convince you of the validity of ethical imperative: that reason is trumped by emotions in these instances.

But I'd say that our hardwired tendency to be "ethical" can manifest in very different ways. Most of us feel various "thou shalt nots." We don't attack strangers. But even here we can imagine sexual, racial, economic, and political otherness serving as justification (in some minds) for such an attack. So there's a sort of consciously directed (ideologically) direct energy that can dominate hardwired empathy. In the name of our principles we dehumanize. We dry our eyes and strike righteously. So I'd read the situation you react to above in terms of cognitive dissonance. Particle is navigating the tensions between his beliefs. He's just using "emotions" perhaps as a name for what are really traditional investments incompatible with his more Nietzschean thoughts. There's also a tendency to understand such thoughts in terms of Shakespearean villains (like Edmund), as if consciousness of terrible freedom automatically demands a dog-eat-eat morality. In other words, freedom is misunderstood as a duty to do "evil." I think this is solved by recognizing that social norms are largely the expression of the selfishness we all have in common. For instance, we all want to do different things with our freedom, but we mostly value freedom in general. So you leave me alone and I'll leave you alone. I'll tolerate your distasteful differences as a price worth paying to enjoy my own in peace.