This is a psychology and self-help word for the sense that you have become the person you had it in you to be — you have used your abilities, done work that matters to you, and lived in a way that matches what you really want, not only what other people expected. It is not just a better job or more money, though work can be part of it; it is the feeling that your life has filled out. The psychologist Abraham Maslow made this sense famous: he put it at the top of his picture of human needs, after food, safety, love, and respect. You will meet it in textbooks, talks, and rather grand personal writing. British spelling often has an s: self-actualisation. Though actually, people use it loosely — almost anyone who feels they 'found themselves' may claim it — so it can sound bigger than the ordinary good season it is describing.