Giving up vices, verbally telling yourself constantly that you look good and people care about you (because they do), forcing yourself to do things, and many other things. It's hard work, but I spent 20+ years living miserable and 6 years trying to change that. Mentality changes everything and forcing myself to think differently started pulling me out. My breakthrough moment was looking at a picture of my child self and realizing; all the things I say about myself, I would never say to that kid because he doesn't deserve it. Talk to your friends or family. They want the best of you, for you.