Hermes, thanks for your kind words and detailed feedback. For the Stoics there is no such thing as virtue above our happiness. Happiness is a life of virtue. (Part of the problem is that the Greek word is eudaimonia, which doesn’t really translate to happiness, but to something like “the life worth living.”)
The Stoics were clear on what “following nature” meant for them: to use reason to improve human society. Which is also their definition of virtue. You are correct that something is missing from the straightforward conception of human nature: the Stoics didn’t think that whatever comes natural to human beings is therefore good. That is a basic logical fallacy. Rather, they inserted a clearly normative sense in their analysis of human nature: the best aspects of it are reason and sociability. These are axiomatic in Stoicism, they cannot be further defended. But that’s not a weakness of Stoicism: any logically coherent system starts with axioms. Which, of course, one is free to reject.
“Following the facts” is Larry Becker’s reframing of Stoicism, not mine. It converges on my thesis once we add Becker’s thesis that human beings are naturally “virtuous” (meaning, pro-social). A thesis with which I agree, based on contemporary research in comparative anthropology and primatology.
The third definition is from Seneca, and it’s not just that virtue is reason, but “right reason.” Again, there is a normative aspect embedded into it. Right reason, for Seneca, is the sort of reason that leads us to work together as a cosmopolis, to improve the whole human lot. So it converges on the other two.
The result is far from being nihilistic. However, you are right that, in a sense, we work for our personal well-being. The Stoics see that as inextricably connected to the well-being of other people, so it is fair to say that Stoicism is a philosophy of enlightened self-interest.