A mission becomes real when it can outgrow its founder.

A mission that depends on its founder is really a preference with staff. It becomes real at the point where other people would carry it forward without you, because they believe in the thing rather than in you. That transition usually feels like a loss of control, and it is the only route to something that lasts.

Ask whether anyone else could articulate the purpose as well as you can. If not, that is the work: making the reasoning shared rather than personal. Explain it until other people can explain it back.