If you define yourself by your work, then you have reversed the order of things. You're serving your career rather than having your career serve you. Even the simplest job exists to serve you--for income and, if possible, for fulfillment. There are people who have very impressive careers and make lots of money, but they're motivated by serving the career rather than letting the career serve them. They are driven to perform rather than experience, accomplish and be fulfilled. They are obsessed with how they appear and don't think about how they are. If the standard of mental health is inner peace, and lack of internal conflict, then this clearly isn't healthy. You can't look at somebody else and determine, like eye or hair color, whether he drives his career or is driven by it. It's an introspective question. Yet it's still real, and in fact makes all the difference.