I'd vote nobody. No in the series actually know how/why the Curse of Ymir works, so how would they go about breaking it? It's vaguely possible that Ymir Fritz knows how to break it, but if she did, she didn't bother stopping her own death in the distance past.
I have a theory that Ymir had a grudge against the people and the world causing all titan shifters to live 13 years. Also we don’t know much about how Ymir got her powers(although it was pretty vague ngl)