If Meg hadn't continued to dig deeper and twist the knives of Jo's pain and everything she was suddenly trying to deal with, Jo might have been able to keep her temper from exploding in the way that it did. But the pushing at her that the boys had moved on and were having good lives -- that she was a part of -- pushed Jo beyond all control.
By the time Meg called her a princess, Jo was doing a spin kick, hopefully catching Meg's chin with the edge of her boot. Jo wasn't even going for finesse. She just wanted this bitch to hurt like fuck. Hurt like she did.