How can I make a smooth transition of a protagonist without feeling it's too rushed or too cliché?

I’m working on developing my protagonist, who starts out as a thoughtful and scholarly college student. Over time, I want him to evolve into a more arrogant, douchbag character. This shift will be triggered by the emotional fallout of losing a mid-semester exam to a friend he always assumed was dimwitted and didn’t stand a chance. I’m struggling with how to handle this transformation in a way that feels natural and avoids coming across as too cliché or rushed.