"No one knows you like you know yourself"

Doesn't that saying just get old. What really confuses me is how do you know yourself or who you are? When think about it, I'm not to sure I know myself. I mean, how can you tell? One day I will know who I am and then the next day I think I'm the total opposite. It's really hard to find myself. I have boundaries and I know what I want(sometimes). But, I don't know who I am. Sometimes I wish I could fast forward through some of the little things. Just answer this, "How do you know who you are?"