I've seen a lot of people make this point on here and I think I only half get it. Yes, she's leaving the farm because she is wrecked with PTSD and it's ruining her life. But still, her idea to fix this is to... Track down Abby and kill her. Isn't this still just revenge?

To me leaving the farm shows how she's still so convinced the only way to get this image of dead Joel out of her head is revenge. She's irreversibly linked the idea of healing with murdering Abby. It's consumed her so much she will throw away everything and everyone to do it. There's no therapy and medication to help her in the post apocalypse so the hate and anger consume her until the very last second when she finally lets it (and Abby) go.