Self-possession is the calm itself — the quality of staying sure and in control of your feelings when something difficult or unexpected happens. People use it with approval: to keep your self-possession is to keep your dignity. You will meet it in writing more than in casual chat; in speech, people often say she stayed calm instead. Though actually, losing your self-possession is not the same as being sad. It means the feelings spilled out in a way you did not choose — tears you could not stop, a snap of anger, a blank freeze. The word looks like possession, which usually means the things you own. This is a different possession: you have yourself.