"Do what makes you happy" is probably it. Expanded upon, it goes more like: "It's your life. You will always and forever be the center of your universe. Not even in an egotistical way, it's just a fact. You are the one person who will always be a constant in your life. Your own well-being should be put first, and that includes your mental state. People tend to forget that. It's good to be kind and care about and for others, but don't forget about yourself. Do what makes you happy."
Another might be "strive to be the person you dream of being". Or an easier version, "try to be a person you'd like to be friends with". But yeah "do what makes you happy" is probably the best one I've got.