I don’t think she stabbed herself. From what I saw of the set video, she just raises the dagger in the air and then disappears. As for Cora, she was probably just theorizing what would happen.
Well, whatever happens, it sounds like Emma is sacrificing herself to save everyone else from darkness. Does that mean Emma herself takes on the darkness by adding her name to the dagger and becoming the next dark one? It could be. Josh Dallas hinted at something terrible happening, and said it would be distressing to fans. What else could Josh Dallas mean by “it’s the worst thing that could possibly happen” in reference to who the big bad of S5 is unless he means Emma? I predict she adds her own name to dagger to become the next big bad of S5.
"That’s how you know you’ve really got a home. When you leave it, there’s this feeling that you can’t shake. You just miss it." Neal Cassidy