Christa is strong. She can hold herself together. She's seen her comrades and friends die, countless times. She knows the risks. She's prepared to die, a little too eager, perhaps, in some cases.
Ymir showed her how to be strong. Many times she would have been lost without the taller girl, or even dead. Ymir was there the one time Christa cried herself to sleep. Ymir was there at Trost, witnessing the horror but giving her an anchor. Ymir was… always there, cracking jokes at inappropriate times, full of sarcastic remarks and witty statements.
Ymir was there when Christa became Historia, her promise to Ymir fulfilled. Historia watched over Ymir as she was recovering, refusing to leave her side. Ymir often called Christa a goddess, but the roles reversed when Ymir saved Christa and everyone else from the tower. Ymir, the girl who lived for herself. Ymir, who would have selfishly ran away and saved her own skin. If Historia truly was a goddess, Ymir was a better one.
Then they took her. They took Ymir away and there was nothing Historia could do to stop them. But Historia is brave. Historia goes after them to rescue Ymir. Historia is not afraid when Ymir's Titan form swallows her. Historia is not afraid to live for herself with Ymir by her side.
Historia is not strong when Ymir leaves her. She cannot hold herself together, shaking and crying in her anger. Ymir taught Christa many things, but she didn't prepare her to be alone again.
I'm not afraid, Historia recalls herself saying. As long as you're with me.
…
Ymir is young when she willingly sacrifices herself to save others. It's her responsibility. So she does it without complaint, without hesitation. Then it is dark, there is no more sun, there is no more anything. She is lost, afraid, alone. She is like this for decades. Hopeless, not in touch with herself, wandering.
When Ymir wakes up from her nightmare, she is blinded by the sun. And she is shaking, not believing her reality. Every time she goes to sleep, she fears of ending up in that hell again. This is what makes everything else so easy. It is not until the first night at the barracks, as Ymir sees Christa fall asleep peacefully, that she is able to rest.