NightMun
During a run-in with a monster, Dipper is left beat up and out of breath. But when Mabel steps in to help, she just causes more damage, irritating him until he can't take it anymore. He ends up snapping her "fun stick", as well as her feelings. And when things go downhill from there, Dipper wonders if their relationship will ever be repaired.
This is some random story I never thought I'd publish. Yes, I know Dipper is a bit, over exaggerated, but it's not bad. Just, blame Bill. It's his fault. Also, Gravity Falls belongs to Disney and Alex Hirsch, but the story is mine, so please don't copy or steal.