Today, I promise myself, will be FOMO-free. No worrying about what others are doing. No stressing about what else I could be doing. No second-guessing the invites I've declined. Me. Today. FOMO-free.

Just one last look at Facebook.

Big mistake. My best writer friend had invited me to breakfast (I said no because I had to work), and now I see she's sharing pancakes with a woman whose friendship I've been pursuing (silently, virtually, of course) for months, partly because I admire how she matches her earrings to her eyeliner and partly because I love her Jessica Simpson imitation (3,492 YouTube hits, 22 of them mine), but mostly because she could help me land a great new gig.

But there they are, and here I am, feeling the first familiar sparks of FOMO. Since I can't live-Photoshop myself into their breakfast, I settle for the healing balm of writing an envy-inspiring tweet: "Costa Rica cabin for holidays! Surf's up!" (Never mind that I can barely swim.) Then I send a Facebook friend request to a different potential work connection I've been too shy to approach. As I await his response, salvation in the form of a yoga workshop invite for this afternoon appears in my feed. This might be what finally helps me focus. How could I miss it?

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The Urge to Be Everywhere at Once

The feeling of FOMO has been around since the first caveman wondered if perhaps he should be out hunting dinner instead of siring an heir. At its root, the phenomenon is a healthy response to variety and choice. But with today's nearly infinite options, it has morphed into something far more complex. Now, FOMO (which nearly 70 percent of adults experience, according to a survey done by marketing communications firm JWT) can feel like an uncontrollable yearning to be two or more places at once, fueled by the fear that missing out on something could put a dent in your happiness, status, or career. "FOMO happens when we invalidate the experience we're having because we're obsessed with the ones we're not having," says psychologist Arnie Kozak, Ph.D., author of Wild Chickens and Petty Tyrants: 108 Metaphors for Mindfulness.

Social media is kerosene on FOMO's fire. Prior to sites like Facebook, a yearly holiday card from an old friend might have made you wonder (or feel angsty about) why you passed up ski-bumming in Telluride or leaving cube life to teach English in Vietnam. But now that you can see your connections' lives in real time, you are theoretically always missing something--a party, a trip, a new career opportunity.

Reined-in cases of FOMO don't go much beyond good fodder for half-serious griping (see The Office's Mindy Kaling's hilarious tome, Is Everyone Hanging Out Without Me?). But a true predilection can result in physical strain if, say, you hit yet another late-night soiree in spite of being bone tired (just so you're not the one who "wasn't there that time"). It can also trigger emotional turmoil in the form of uncertainty, regret, or envy, says psychologist Sherry Turkle, Ph.D., of the Initiative on Technology and Self at MIT.

Such emotions can lead to resentment--toward yourself and others--as well as feelings of dissatisfaction, anxiety, and unworthiness. And these, in turn, can prompt you toward a type of defensive sugarcoating that has become all too recognizable online. Take my Costa Rica post, which was basically my way of showing my social circle (and, of course, myself) that I don't need to be at some breakfast meeting to be fabulous. And perhaps it was no coincidence that minutes after that tweet went live, a housebound new-mom friend posted, "There's nothing better than raising my adorable little Caleb." If I can't be there having fun, the thinking goes, I'll show I'm having more fun right here.

"FOMO prompts people to use social media to present enviable versions of themselves," says Turkle, and this behavior can seriously mess with your head: "One of the weirdest things about FOMO is that people find it hard to live up to not only the images projected by others but also the image they've presented of themselves."

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