Work On Your Mental Health

This can't be stressed enough. Face your problems and don't push them aside, because a small patch of snow can become an avalanche. Find whatever feels best for you whether it is meditation, taking long walks alone, going to therapy or even speaking to your friends/family about it. Find a philosophy that suits you and try to practice it everyday. Happiness as a state doesn't exist, you're not going to magically feel happy forever just because you got married or got your degree, you have to constantly work hard for it.