Self-confidence is helpful alongside agency, but I think it's a totally different concept.

Agency is about self-awareness and realizing that you have a choice. Lack of agency means going along with the default, societal script or what is handed to you even if it is not a good option for you.

One example is dropping out of a university program, even if you are capable of graduating. There is a strong social expectation that you finish, but maybe you don't like it, or your time is better spent elsewhere. Agency is that realization that you can choose.