How does one become “The best version of themselves”?

I hear it often, but it doesn’t make sense to me because what is my “best”?

Is it to be the happy and fulfilled in life, is it a state of mind, or is it being whatever is best for society?

If it’s any of those then I would be labeled as the “worst” kind of person

What would even be “better” in my situation, because I’m a negative person in general.