1. I can't leave my team hanging in the middle of this big project. There's rarely a "good" time to leave a job. If you wait for the perfect time, you might be waiting forever. And even if you leave in the middle of an important project, your team and organization will manage to get by, no matter how bad the timing. And no reasonable person will blame you for the timing; this is just how this stuff works, and most people understand that. So job search when you feel ready, and then once you accept an offer, give as much notice as you can, leave your work in good order and provide thorough documentation for your replacement. That's all you can do, and all that is expected of you.
2. My job, despite its flaws, is familiar to me, and I feel anxious or sad about leaving. Even when you know leaving is right, leaving the familiar and going somewhere new can be hard; it's the unknown, you don't have a routine there, and it can be daunting. As bad as a job might be, there are still usually some things that you like or at least feel really comfortable with, even if it's just the physical space you work in or your routine of getting a sandwich from the deli downstairs every afternoon. It's normal to feel this way; the key is just not to let it get in the way of your making good decisions for yourself.
3. My manager has really gone to bat to keep me. Your manager might have pushed hard to get you a raise, a promotion, or better assignments, even using her own political capital to do so. But this doesn't obligate you to stay forever. It does obligate you to speak up if she's in the midst of pushing you to get you something when you know you won't be around in a few months – but if it's already happened, and especially if it was a year or more ago, you don't need to feel tethered to a job you're ready to leave. If your manager is good enough to inspire this kind of loyalty, she's going to understand that people move on.
4. In this market, it will be too hard to find a new job. Job searching in this market is tough. But people are getting hired every day, and fearing that you won't be able to is no reason not to even try. Besides, if you lost your job tomorrow, you'd have to find a new one, no matter how tough the market, right? So put together a strong résumé and cover letter, talk to your network, start throwing your hat into the ring for jobs that interest you and see what comes of it.
5. If I go somewhere new, I'll lose the status and respect that I have at my current job. When you've established yourself in one organization, and enjoy the credibility and respect that accompany everyone knowing your work, it can be hard to imagine having to build that all over again somewhere new. But there's also a price for staying where you are when you know you really should be moving on – whether it's staying in a situation with a difficult boss, or a job where you can no longer advance, or with an organization that won't pay you what you're worth. And you presumably earned that status and respect at your current job, and you'll earn it at a new place too. (And having that respect from a whole new group of people is a great move for your career.)
Alison Green writes the popular Ask a Manager blog, where she dispenses advice on career, job search, and management issues. She's also the co-author of Managing to Change the World: The Nonprofit Manager's Guide to Getting Results, and former chief of staff of a successful nonprofit organization, where she oversaw day-to-day staff management, hiring, firing, and employee development.