Erin survived his injuries by turning INTO a titan, which regenerated his own body. Ymir seems to have sustained those injuries to her body WHILE a titan, so unless she turns into a titan again, they wouldn't heal.

That would seem to be why attacking the neck is how you kill a titan. The fur titan even commented that "They know we live in the nape." So obviously every titan has a little human body in it's neck, and if you hit that body, you do real lasting harm.

Quote Originally Posted by KrayZ33 View Post
I mean, you don't smile when you are dead?
People do that all the time in anime.