FOMO happens when you're not in the moment when you don't do what you feel compelled to do (but for whatever reason, you rationalise not doing the thing).
I tend to land somewhere between Socrates and Epicurus.
I understand that not all my decisions would pass the high water mark that Socrates' critical thinking require. And at the same time, most of my decisions are not Epicurean at all. I work more than play, even though I would tell you that my work is often play (at least to some extent).
All of this is future states to some extent, though. We take a view of self and the future that is more about tomorrow than what it is about today. Ego/Self propels us forward to a better state. Nothing wrong with that. And when we do that blindly, we've skipped over what is (and - in hindsight - was) important today at this moment.
I suspect there is no way to know with absolute conviction today that you have lived a good life. I know that a good life is likely a combination of good moments.
Neil Young said, "it's better to burn out than to fade away". Kurt Cobain quoted this in his suicide note.
I bet both would concur that they only had a single life to live. (And that they both lived through incredible moments.)
As binary as the outcomes seem, we only need to know that the spectrum is so vast when we place ourselves on that spectrum.
You live life (only) once.