In my case — and, from what I can tell, in many others too — this malaise arises out of cognitive dissonance. That is to say, there's some bothersome mismatch between how things are and how I'm thinking about them. My crude hypothesis is that, if your conception of your life and the ways you actually use your time drift too far out of line, you basically just hear the Emergency Broadcast System crunch in 3D SensurroundTM over and over again until you either fall in line or crawl under some form of opiate.
"He's not a bad guy," I've heard girls repeatedly insist about their boyfriend-shaped disasters, "not on the inside. He just does bad things." But are the two separable? What imaginable argument could make the actions not just distinct from but non-identical to the person? What or where, exactly, is the "inside"? We all see right through this when someone else spouts it in what seems like a trivial context. Your mileage may vary, but when it comes to a topic like what my very life is about, the blinders are a tad harder to get off.
It's pretty easy to produce a hand-wavey explanation of why your life is "about" something other than what you actually spend it on every day. It hastens a hoped-for subject change in conversation, perhaps, but it does nothing to close the gap between your real life and your declared one. Clichéd example: the barista who claims to "really" be a novelist. Let's say he spends eight hours a day pouring macchiatos. If he is, indeed, really a novelist, he must spend more than eight hours a day writing novels, and his time behind the latte bar is secondary in quantity as well as secondary in perception. If he spends more time pouring macchiatos than anything else, well, he's really a barista.
There's nothing the matter with really being a barista. I tend to agree with observers who are weirded out by that fact that so few service workers they meet seem to genuinely inhabit their jobs. (Then again, I'm even more weirded out when I meet a service worker who does genuinely inhabit their job.) The wrongness comes, again, when words and thought clash with reality. This is the wince you see when you ask a new friend what they do and they say, "Well, I play music, but..." or "I'm a filmmaker, except that right now..." This is the wince you let out when, as the new friend, you say this stuff yourself.
Maybe you wouldn't wince if you said, "I'm a temp, but I mostly browse Wikipedia, Reddit and 4chan. I spend a few hours every week envisioning different scenarios of putting a band together, and sometimes I listen to my favorite songs and think about what it would be like if I had recorded them." Or if you said, "I'm a sales associate at Kinko's, but on my days off I re-read Jane Austen novels and think about how I could write them under different conditions, post the occasional fanfic and half-fill grad school applications every few years." Okay, you'd probably still wince, but it'd at least be a different wince; it wouldn't be the acidic wince of dishonesty.
To the extent that your time gets consumed by writing, and only to that extent, you're a writer. The same goes for time spent rapping and being a rapper, time spent training and being physically fit, time spent watching RSS feeds and being a reader of RSS feeds. This might sound agonizingly self-evident, but it's shockingly easy to forget in the course of day-to-day existence.
You can consider what your life is about to be what you've already accomplished or what you imagine yourself accomplishing in the future, but I find those ways lie madness. What life's about is whatever it's being burnt on, which, in turn, is what you are. A sometimes unpleasant way of framing, perhaps, but it frames clearly and guides you toward more interesting uses of your time. I never could improve myself without clarity.