How do I keep my job from defining who I am?

You know who you are underneath any title, so your work can express you without boxing you in. That is a steady place to stand. The job is something you do, a way you show your gifts to the world, but it is not the whole of you and never was. This matters most when work goes well or badly. When it soars, you do not mistake the praise for your worth. When it stumbles, you do not mistake the setback for your identity. You are the same person in both, because the ground you stand on was never the job in the first place. Keep it that way by staying connected to the parts of your life the job does not touch: people, interests, quiet, things you do for no payoff at all. Tend those. They are the roots that let your work be an expression of you rather than a cage around you.