Most Helpful Girl
It's a strict imagination process. In the first place you should start making a mental list of things/habits/behaviors that he actually does. The most stupid and annoying ones Then you start imagining how this things can destroy your life and the ones around you. You're basically multiplying his flaws until you'll see him like the most worthless and fucked up person in the world. You'll start creating scenarios in which he will lose his temper and become something you'll never imagined he could. And in time you'll start believing yourself so damn much than the reality will seem the imaginary world.