I was hired into my position by a wonderful man who was grooming me to become his successor. I was surprised that when he retired, I was passed over in favor of someone who is much younger and less experienced than me. I feel like I'm training her. Should I start looking for another job?
This situation is more common than you might think. It calls for a good night out with friends to vent your anger and frustration about the unfairness of the world and the lack of appreciation you've received for your hard work.
But then you have to face reality. Once someone has become your boss, you have only two choices: One is to embrace the situation fully and offer to help wherever you can; the second is immediately to start looking for another position. The first option is tricky, because your new boss may perceive you as a threat. You know more than she does, and given your interest in her position, she may question your loyalty. Unless you're prepared to bend over backwards to make your new boss successful, it will be hard for you to continue in your position.
Before you make a definite decision to jump ship, however, consider having a frank discussion with the person who hired your new boss. Ask for honest feedback on why you didn't get the job and what he sees as your future within the company. Depending on what he says, consider asking to leave by mutual agreement—along with a severance package.
Whatever you do, don't get down on your old mentor. He's probably as saddened by the situation as you are. Instead, enlist his support in networking and exploring alternative job options. He's likely to be your number one fan.